If it's plainchant, I'm partial to Pater cuncta. If it's polyphony, well - Tallis' mass for seven voices or a grand Monteverdi mass complete with cornets, sackbuts, and all. Definitely something with an unmistakable ecclesiastical solemnitas.
(You don't want an ordinary ordinary for an occasion such as this.)
I think the Willan (Communion Setting 2, H-1940) would work well in Latin. I think the folds at St. John Cantius were going to look into copyright permission for such a project.
There is also a unison Mass of St. Theresa by Willan - in The New St. Basil Hymnal. I'm working on scanning an accompaniment copy for CCW right now.
Depending on what you use on a regular basis and the ability of your choir, I have found that the Missa Tribus Vocibus by Christoph Dalitz + Credo I (or another) works well for a little added solemnity.
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