>>>Is this a limitation of MOI or Solid Modeling in general?
Are there solid modelers that are more "elastic"?

Most parametric or direct modeling apps will give you the choice to edit features like fillets later on...http://www.youtube.com/watch?v=DH8DFtF-m8k http://www.youtube.com/watch?v=pqE737atPnY

fusion is a good place to start since you use 3ds max (DWG support ;) you can download for free from the labs site, just log in with your max account. From what I heard is gonna be free for Autocad, Inventor users...

BUT be aware is not a friendly as MoI :)