The tradition is 3 or 4 trombones, not trombone solo. And I assume this is really solo, not with accompaniment. which reduces it further. My inclination would be to look at the sacred solo literature instead of the trombone literature, and then do it unaccompanied. Maybe something like "I know that my Reedemer liveth" from Messiah.
There's the obvious sarabande w/o double stops, and then the even more obvious Gregorian melodies, used in another solo cello work together with double-stopped commentary.
Years ago (and I mean years ago!), I wrote an "Entrance Antiphon" (yes, that long ago) on the text "O look at me and be merciful"] with a distinct chorale style to the setting. I had dusted it off in 2002 and set it as the opening (and closing) Chorale of a Trombone Quartet I was writing. The style of the chorale might well be appropriate for four trombones playing at a funeral (there is also a higher transposition for two trumpets and two trombones, and of course there is something approaching the original 4-part vocal setting).
As a side note, Liam might well have sung the original when he was in my choir at St. Thomas Aquinas Church in Charlottesville, Virginia. He can attest that the time frame was (relatively speaking) eons ago!
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