Without consent of the person in the coma prior to them being in a coma, to being tested on while in a coma... I highly doubt someone has done such research (at least legally).
The researchers would have to know you'll be in a coma at a certain time for a certain length of time, and the only way to know that is to induce one, and I'm not sure how safe that is, can't imagine it's safe to do.
And for patients suffering from a coma due to disease/injury, I don't imagine it's ethical to use them for research either, unless it's for something to potentially cure or treat their condition.