Sorry I read your question too fast
Honestly, I don't know the answer to the question but I really don't understand how the protein could lose much of its effectiveness just from being premixed. If you think about it, all those ready to drink cans are premixed also but just have some ingredients to preserve them.
The only thing I was able to find was some people making references to certain chemicals being broke down by letting the protein sit in a liquid for a long time. Most of these references were to protein powders that sat in liquid for more than 24 hrs, so I think you should be fine if you just want to have premade and chilled for a short time. Protein shakes do go nasty fast and start to stink if they are left too long though.