I think a factory is for when the caller knows what they want. Your case is different.

I find out which subclasses are available by scanning all or part of the @INC path list for modules in a given namespace. I do this once, as the app starts. Note that big @INC scans can be time prohibitive. In that case, I look to see which directory my parent module lives in by checking %INC, then look there for my children, making the big assumption that they get installed to the same place as the parent. It works for Bigtop's tentmaker.

Phil