tricky - it might not be any easier, but you could load the 'locked' swf into one you control, then spider through its objects until you hit TextBox, using some for...in loops - it'd be a long, arduous process to map them out then change them, especially if the previous developer didn't name things in a helpful way, but if it's a fairly simple .swf then it might be too bad...
Also, there's a mac-only utility for decompiling swfs that I remember a coworker swearing by, but I don't recall the name... anybody?