15. In sacred processions conducted according to the liturgical books, only the language prescribed or permitted by these books should be used. In other processions, held as private devotions, the language more suited to the faithful may be used. (De musica sacra et sacra liturgia)
Although only Latin may be used for the Corpus Christi procession, there is no stipulation that only Gregorian chant may be sung. Your congregation probably at least knows a "metrical" Tantum ergo, O salutaris, and perhaps Panis angelicus in which they could participate. I share your concerns about outdoor processions with a small schola. And here in Phoenix, it is already miserably hot even with the procession scheduled following the first Mass of the day!21. In processions outside the church the Ordinary may give permission for a band, provided no profane pieces be executed. (Tra le sollecitudini)
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