Yes, but then you can't print hash tables in a way that is also readable, and unambiguous with a list.
Right now:
arc> (= codes (obj "Boston" 'bos "San Francisco" 'sfo "Paris" 'cdg))
#hash(("Boston" . bos) ("Paris" . cdg) ("San Francisco" . sfo))
Well, I'm kinda ashamed, but I just found out that define in scheme does just that. If you define a variable inside a block, the variable belongs to that block. Does def in arc work like that also? (I don't have arc in this computer, so I can't test it)