Simon,
Are you perhaps referring to 'hymn preludes', composed by members of MIMF?
-- or --
'Improvisations' based upon known hymn tunes that are used in many countries?
I do improvisations that are my own creations, but not based upon any known (that I am aware of) published tune.
I have also taken established hymn tune melodies and fashioned them into a meditative piece for service music filler. Unfortunately, they were 'on the spot' things and not recorded (I now have a digital recorder nearby) so I am having to re-create them in notation software from memory,

and those are work in progress, far from being ready, imho, from public disecting.

This will be a fun and interesting thread to follow and see how it develops.
Lars