As a player of plucked and hammered instruments, I have yet to encounter a plainsong melody that doesn’t work wonderfully well on these. Voices, yes, of course are optimal. But there is something about plainsong on an instrument like the hammered dulcimer and its sustained tones that works for me. It might just be my long affinity for the instrument.
I’m finding just a select few people in my new parish exposed to plainsong. But one couple I met with recently raised the issue. They settled on a choir piece and a solo for the prelude. But In Paradisum came up, and they decided I would play it on hammered dulcimer.
As I was preparing a score this afternoon, I was thinking back to the last time I played it–at another funeral back at the campus parish. One elderly couple were especially fond of chant, and on the occasions I pulled something out for Mass or Reconciliation, they would often comment on it. They always knew the tune, which probably says more about my limited repertoire of twenty-something than anything else.
Catholic Sensibility 
Don’t forget the Salve Regina – simple tone. Very widely recognized in the Catholic memory bank because something people used to hear frequently.
The succession of three tropes at the end is particularly lovely, of course. I remember a modern choral piece that used them as the foundational device for an entire set of Advent verses/antiphons.
Insert Big Lebowski quote here: https://www.youtube.com/watch?v=z2rZHT_SrzA
If they’ve ever heard Duruflé’s setting of the Requiem, then they’ve heard the chant (and a bunch of others.) It gets used in movies quite a bit, as well. (As does Fauré’s setting. The scene of the main characters traveling into an empty road into Manchester in “28 Days” is stunning/chilling.) If memory serves, Fauré’s “In Paradisum” is also based on the chant, but less overtly. Been awhile since I looked at that score.
In Paradisum is one of my favorite chant pieces – and I’d like it sung at my funeral. I also find Duruflé’s and Faure’s pieces moving – especially Duruflé. I would love to hear it on hammered dulcimer!