On paper, it's a good idea.
Thematically it's an awful idea.
In practice just as bad since the wetness would create humidity inside the raft and everything would end up all foggy inside from the moisture and body heat.
How wet you get is really just a luck of the draw anyway. I have gone on Grizzly with near nothing wetness and othertimes where I was mildly wet (No ride on GRR for me has been as wet as a ride on Splash)
Additionally, even in winter it's not THAT cold THAT often in Southern California that such drastic measures would be needed, IMO.
The best solution, which has been in place since the park opened is that Disney does sell ponchos and offers FREE 2-hour lockers.