Franck has a bunch of really easy pieces in different keys.. I think I got them from this CD: http://www.halleonard.com/product/viewproduct.do?itemid=220559&lid=0&subsiteid=63&;

If your hymn were in F you'd use one of the pieces in F Major/Minor.
If your hymn were in G you'd use one of the pieces in G Major/Minor.

When I was first starting out I used these often. Haven't picked them up in forever though, and perhaps I should!

Thanks, Gavin. I certainly keep working on my improvisation, but time at the instrument prohibits me from digging into it more (also, lack of instruction from someone better than me).

Matthewj, I've tried similar things, like Rossini's The Liturgical Organist and Shenk's Brief Orisons. The Shenk pieces are fairly nice, and I'm not so nuts about the Rossini collection. That type of thing is exactly what I was looking for, though. Thanks!

Edit: is this the collection you are speaking of?

Yes. The F major/minor ones are my favorites.

Thirty Five Miniatures For Organ
Flor Peeters, Op. 55

Modal, in fact, Dedicated to my dear friend Theodore N. Marier.

I keep The Hymnal, 1940 nearby. I use those excellent hymns as filler, when needed.

Another nice resource is Charles Callahan's "Chant" volumes 1 and 2. Also, don't forget about the Eighty Chorale Preludes published by Peters. Lots of good stuff in there.

How often do you practice? Perhaps you could set aside 15 or 20 minutes of your practice time just letting your hands and ideas freely happen, mistakes, good sounds, awful sounds and all. All the above advice is really good, but you should accustom yourself to being cautiously bold in discovering how to construct a sensible musical interlude or short composition. A knowledge of basic harmonic progression would be most helpful, but shouldn't define your limits. Also, analysis of short composed works will give you hints of what you can do on your own. As Gavin suggests, don't be ashamed to start with a mere solo line, or a two voiced effort. (And! It doesn't necessarily have to be soft=innocuous.) Make simple two or three voice improvisations on a chant cantus firmus part of your daily regimen. Never ever put yourself down - accept where you're at and build on that.
Also: look into some of the earlier mass and magnificat versets and suites by Renaissance and Baroque composers... Boyvin, Frescobaldi, Pachelbel, etc.... these are very often rather brief and would make very nice 'interludes'. Too, if they are too long you can nicely tailor them as needed... as a contemporary organist would have done.
Numerous organ methods have chapters on improvisation. You might investigate these. Do you belong to the AGO? Its monthly journal, The American Organist, always has an how-to article on basic improvisation. And, interaction with other organists could be most rewarding. Never miss an opportunity to learn by hearing others in recital or service playing, especially Episcopalians.

Peeters' work is generally modal rather than 'keyed'--thus very fitting for Catholic church work.

Whenever I need filler, I keep the Graduale Romanum handy and improvise on the Introit of the day, usually, making up the left hand and pedal as I go, and if I'm feeling it, play the melody on the pedals. If it worked for Widor and Dupre it's good enough for me and my village. Not that I'm improvising symphonies or anything... I do change registration often. OF, btw.

2 weeks before I started at my parish, I decided to stop by to see what the previous organist was doing for music, I suffered through "Amazing Grace" in 5 different keys as Holy Communion was finishing up. Barf.

Marcel Dupre's Course of Improvisation (in 2 volumes) is worth the effort, though it is rather difficult at times. Many good things to learn from it.

Pachelbel. His Fugues on the Magnificat (Published by Dover) are good and short; also some Preludes, Toccatas, Fugues (Published by Dover as "Pachelbel, Organ Works"); also Hexachordum Apollinis.

There are some Intonazione by Frescobaldi and Gabrielli (IMSLP) that are nice as short interludes. Also try looking for Antiphons and Hymn Versets for the Office (especially French ones).

I agree with Salieri on the Frescobaldi/Gabrieli intonazioni and versets. Late 16th and early 17th century organ books are full of ready-made short pieces that can be used as interludes. The Corpus of Early Keyboard Music (CEKM) volumes are where I find a lot of these. Many of the pieces could be a bit long for what you are describing, but they are written in sections so that you can stop when you want to. As someone else said, metric hymns are also good; if you need "really short" you can start one of these somewhere in the middle and end up with a nice final cadence. As was also suggested, I keep a folder of this type of thing so I can pull any of them out in seconds.

Overheard from a jazz player:
When improvising, remember- you're never more than a half-step away from the right note.

I keep a copy of The Hymnal 1940 for use when I need a little more music to fill time. That's the best hymnal of all time. For example, the pastor wants something played until he walks to the side of the altar for hand washing.

Don't have that one. Maybe the best Catholic hymnal, since the ordinariate is using it.