When I used to seal model airplanes with nitrate dope (which is very similar to collodion), we used to add a few drops of castor oil to a batch of dope that needed to remain flexible or not shrink as it cured. I expect something similar to this (generically, a "plasticizer") would help a lot if you wanted to try collodion on acetate or other flexible base. However, you'll probably also find you need to sub the base, or use a textured material (like drafting acetate, which is frosted on one side). And I expect acetate would be a bad choice for collodion, since many of the solvents that dissolve cellulose nitrate also dissolve cellulose acetate.
Oddly, *paper* takes nitrate dope well, so might accept collodion well also...