What kind of percussion instruments? There is a lot of difference between kettle drums and tambourines. Of course, you know that, but the point is that appropriate percussion along with a small orchestra or a consort of brass instruments on a great feast is quite a distance from the disgrace of tambourines and snare drums accompanying happy-clappy music on average Sundays.

I'm sure someone here has the documentation and answers you are seeking (I haven't needed it since, thankfully, I only encounter those odious djembe and rainsticks in Mass when on vacation), but in the meantime, I echo the request for more context.

As a percussion major in my undergrad, I am amused every time I see anyone playing any percussion instrument during Mass. I spent hundreds of hours on tambourine, trying to get it right for all those difficult licks in Slavonic Dances, Pini di Roma, Carmen, Scheherezade---and it all gets reduced to backbeat banging in some places. Not to mention the hundreds of hours alone for the cymbal crashes in the Rachmaninov Piano Concerto #2; takes tremendous strength to play so softly. Too bad!

Here is the clarification:
Percussion is not used at my parish in an orchestral context, unless you consider two guitars, a lady who can't carry a tune, and a few assorted singers to be an "orchestra". Lol. They mostly use tambourines, rainsticks, triangles, bongo drums, and assorted related percussion to "accompany" songs such as "Fly like a bird" and similar pieces from OCP.

Although that clarifies the circumstances, I am more concerned at this point with determining the actual weight of the bishop's informal oral statements with regards to this discussion, as they are the only grounds for allowing such instruments in our diocese that I know of. If someone is familiar with a section from canon law or an official document of the church which would apply in this case, I would love to know about it.

Mike Joncas' little tome, FROM SACRED MUSIC TO LITURGICAL SONG, is a handy little digest of both legislative and advisory documents. Anthony Ruff's SACRED MUSIC AND LITURGICAL REFORM also deals with issues of what supersedes or abrogates what within these concerns. And as per usual, there is little consensus in their conclusions, no doubt due to purposeful ambiguity and translation issues. rogue63's advice wise is.

More clarification: I am a music minister at my parish. We do not currently have a paid music director on the staff, so the pastor is the only one who can decide these things right now.
Thankfully, my parish is a step ahead of some, in that if I can quote a document showing the official position of the church, the pastor is willing to listen and even act.
In light of this, do any of you know of specific supporting quotes that might be useful in this discussion?

',,,according to longstanding local usage, insofar as these are truly suitable for sacred use, or can be made suitable.'

This is an opening large enough to drive that truck through, isn't it. Like most such pronouncements, it sounds good but means nothing, nothing at all, except what any given person wishes it to mean.

It is significant, though, that the single instrument mentioned in the documents of Vatican II is the organ. One would never know it in the majority of churches that use everything but the organ.

More clarification: I am a music minister at my parish.

One of many, or the music director?

I can't imagine: 1. Any bishop having the time to answer a question like this and 2. any ruling that would make any difference in a local parish.

The only percussion in my loft on Sunday mornings may be me banging the head of a hippie musician against the console case - in time and observing all dynamic markings, of course. :-)

No hippie, even vote Republican. It's those liberal guitar hippies that want to express themselves in the liturgy that earn my ire. LOL.

In all seriousness, getting across the idea that the liturgy is not about individual likes and dislikes, nor about how one "feels" is one of the more difficult aspects of this job.

Number 1: your priest is the boss; do what he asks you to do. Number 2: informal oral statements made by any person of authority are not official, and are therefore not able to be considered as granting nor denial of permission, especially in this case as you cannot verify the validity of said statements. Long story short: private, unofficial statements made by a person of authority do not constitute an official ruling: they carry no weight. Number 3: consider also that options 3, and 4 of the GIRM require permission from the local Ordinary, yet many of us cannot find any record of such permission being granted, although option 4 is the most frequently used, seemingly without permission.

My suggestion: write a letter to the bishop asking for clarification on the matter. Make sure that you cite any references from Church documents that you feel are relevant. (BTW: This is a safe recommendation because you're not pushing for any specific result, per se, you are asking the bishop to clarify his position on the issue so that his order can be followed more effectively). If it is indeed true that the bishop granted permission for instruments such as the ones you named to be used in the manner they are being used, then you must accept that answer. However, especially if this specific group is trying to state that the bishop specifically granted them special permission (i.e. the bishop privately told them that at their specific church, they can use percussion), it seems highly implausible, and I would be suspicious of the validity of such statements.

This letter will either go completely unanswered or will get a rather patronizing response. I am willing to bet money that your bishop doesn't see any specific problem with percussion of any kind (or any style of music) being used in liturgy, and that he thinks the requirement for specific approval is a silly anachronism --- if he even knows about the rule at all.

I imagine the original referenced conversation went something like:
"Bishop! You should check out what we're doing at 6pm over at St. Claptrap. Guitars and drums and tambourines. Very active. So participate. Many vibrant."
"That sounds great! Keep up the good work!"

I further imagine the following response.
"Bishop! You got this letter from some guy who wants to know if you have given specific permission to use percussion instruments in liturgy. Should I just ignore it?"
"No. Write him back and say I did that. Something something enculturation, something something Vatican II."
"Ok."
"And put him on the list, will ya."