I can say from personal experience that you used to be able to do this on Live, and you wouldn't have the speedup/slowdown like you did with the new models. Of course, it was an exploit, and was changed later on. I'm not really sure if the check is just server side, or if it is just client side. If there is a server side piece (which I assume there is), you'll probably want to check
zone/spells.cpp. Seeing as how the blocking is Live-like, I don't think it will be changed in the source, although it might be good to have a Rule for it (on by default).