Why not use nitric acid? Won't attack the aluminum.
Heh... why indeed, right? Can get it from chemical suppliers, but if you don't meet their checks, it kinda sucks. Well, you can make it in-situ with sulfuric acid (drain cleaner?) and a nitrate salt. Sulfuric won't attack aluminum either, at least not very quickly, but an oxidizer is needed to attack copper.
Also, tin? What form of tin? Isn't tin a mask against ferric chloride?
What's wrong with conventional masks like... just regular old grease or paint or tape?
Soldermask is rather tenacious as mentioned above, but a chemical stripper (preferably methylene chloride based?) will remove it without affecting the metals.
I suppose even just good old fashioned heat might work, too. Unless there's an organic adhesive bond layer in there already. The supposition is whether anything adverse results (CBN should be stable at red heat, I think? Aluminum will be annealed though, and may be too soft.)
Tim