sorry for my late reply... You should try to get a hold of Phil Lancaster, who is arguably the top coach with respect to flexibility training. Years ago there used to be mailinglist for contortionist where he gave extraordinary detailed advice. I would be interested to read what he has to write on the topic today.
One thing I remember, I used to have a printout but I don't find it anymore, is that he adviced doing calisthenics between stretching sets. Say you are working on your side split: do your 30sec or how long it takes stretch, then do 15-20 JumpingJacks or similar, and repeat the stretch etc. Alternate these till you are ready with your session. The reason he gave, if memory serves me, was that you would not get a s sore as you would without those jumpingjacks. So you could savely continue early and make in consequence larger progress in a shorter time frame, and of course it keeps you warm between sets which is paramount for stretching.