kat, let your heart, their eyes and their words guide you. Play what they request if you can, if not, play something similar. Talk if they want to, talk about what THEY want to talk about, play soft, gentle stuff for those in coma--there is a LOT of evidence that they can hear, it's the last sense to go.
If your heart says "DO IT", then you know there must be a reason.......I want to do it, too, but I am not far enough beyond my mother's death to be able too yet. The last time she was at my house before she died, I spent about 3 hours playing her viola for her......
She alternately beamed and cried.....and asked for more. It was hell on my hands to play viola that long, as it is so much bigger than my violin, but I am so glad I did it. I had planned to "harp her home", but she was only in the hospital one day before she died, and that was my sister's day with her.......