I have two CDs of meditation music, and play live for guided meditations and prayers all the time. If you’re accompanying someone who is speaking a prayer or meditation, the sound source and the approach are both important.
Basically, you want a sustaining element and a few color elements that you can bring in and out. A simple synth pad is a good start. Layer it with a very light electric piano and you’re off and running. Other instruments can work too! I just tend to go with keyboard for this type of thing.
Then you just listen. When the leader is speaking, you hold. In the spaces, you move. If you sense a new theme or direction coming, you can anticipate it, maybe change keys or pick up the pace a bit. If it’s someone you connect with energetically, you can definitely create a dramatic arc that means something. You can also slow down a busy speaker, or subtly encourage someone to get it over with, already! But don’t tell anyone I said so.
Also, most leaders will say that they don’t want hear a recognizable melody because it pulls the ear away. This is not the place to sprinkle in quotes from the jazz repertoire.
FWIW, that’s my approach, but some people do it differently and get on just fine. My CDs are more active and have definite themes and melodies, so that’s a little different from doing it live, but even so, I’ve sold lots of them to people who use them for meditation and yoga at home. One couple told that they use them for . . . well, never mind.