It is my understanding that (both in EF and OF), rubrics proscribing the use of instruments are suspended on Masses of Laetare Sunday. So, theoretically, the organ could be used as normal, an orchestral Mass setting could be employed, etc. I was at St. John Cantius a number of yeas ago on Laetare, and they used an orchestral missa brevis of some sort.

Organ alone for EF, but apparently instruments in general for OF:

The organ only (including the harmonium or reed organ) may be used on the third Sunday of Advent, and the fourth Sunday of Lent, on Thursday of Holy Week during the Mass of Chrism, and during the solemn evening Mass of the Last Supper from the beginning to the end of the hymn Gloria in excelsis Deo (De musica sacra et sacra liturgia 83b)

In Lent the playing of the organ and musical instruments is allowed only to support the singing. Exceptions are Laetare Sunday (Fourth Sunday of Lent), solemnities, and feasts. (GIRM 313)

Someone with a copy of the Ceremonial of Bishops for the new rite could tell us if it spells things out any more explicitly.

It is OF, and it would just be a violin to accompany the Mass Parts and maybe a couple of the hymns. No solo, of course. I don't want to overdo it, but it is a celebration of the parish and an episcopal visit, so I would like to take it up a step or two from a regular Lenten Sunday. From the comments above, it seems to be legit to do this. I do plan to use a keyboard with strings for probably one or two items, as this also seems to be within guidelines. Does anyone strongly disagree?

Semantics aside, certainly there's nothing particularly wrong with a violin doubling the melody, is there Chris?

I do plan to use a keyboard with strings for probably one or two items, as this also seems to be within guidelines.

As regards this, do you mean string stops on an organ? Or string sounds on some sort of keyboard? If the former, great. If the latter, I would say that the use of keyboard strings often comes across as sappy/cheesy, for lack of better words. If your'e using a keyboard and not an organ, it would be best to stick with whatever organ-simulation-sounds it provides you with.

There's nothing wrong with a violin doubling the melody, at least not intrinsically, but in most parishes the violin won't be doing anything other than parallel play (unless it is attached to a microphone, in which case, "a pox upon you!")

One could make the argument-from-need for organ, allowed for in Advent and Lent, but that's not explicitly stated, and requires a stretch.

It doesn't require a stretch. The use of the organ to support the singing is an enumerated exception in the legislation itself—not an interpretation, and not an ambiguous loophole. While De musica sacra does indeed preface the exceptions with the words "However, during the seasons, and days just mentioned," it seems obvious to me that what follows also applies to "all Offices and Masses of the Dead" also mentioned in the previous paragraph (81). This prohibition has been in place since at least the Caeremoniale Episcoporum of 1752; it is not a Pius XII novelty. What was new is the strongly worded absolute prohibition of organ and instruments for non-liturgical services on Good Friday (84). It is also apparent from that same paragraph that the period between the Glorias of Holy Thursday and the Easter Vigil is the only time the organ is prohibited even as accompaniment, but the local Ordinary has the authority to regulate its use more precisely. (Do any of them actually do so nowadays?) Apart from local custom, the rule at such Masses is that the organ is silent when there is no singing, which I think should be interpreted strictly—no introductions or interludes.

The only time we have a right to be disobedient is when by rendering false obedience we would be complicit in something sinful and therefore disobedient to a higher authority. If you don't have the permission of the Ordinary for an orchestral Requiem Mass, you shouldn't do it in the liturgy. You could have a concert performance in church, but that also requires the Ordinary's permission. Yes, such works were composed for actual liturgical use, but unless you're somewhere where orchestral Requiems were historically customary, incorporating them into the Mass has at least some element of theater about it—not necessarily a bad thing, but we musicians need to be aware of possibly giving the impression that we're playing make-believe and pretending we're in Vienna or Salzburg—and perhaps exhausting our music budget by doing so. As you said, "pulling off any orchestral Mass takes some doing." The liturgical, pastoral, and monetary resources and needs of the community should be carefully considered even if your bishop permits it. I'm not sure I follow your reasoning concerning what's objectionable about a violin solo (presumably accompanied by organ) as opposed to an organ solo at a Mass where instruments are permitted.