I did hot yoga before donation and asked about it when I was going through testing. The nephrologist and surgeon but said it should be fine, just stay hydrated (although I am not really sure either of them really knew what I was talking about.) I actually don't do it now as often as I would like, but I am very conscious about drinking more than usual water during the day before yoga and lots during and after. I also run, and now that it is hot I do the same, drink more before and I take a water belt with me to drink during the run if it is more than 50 minutes and hot out.
Namaste :-)
Julie