Have you considered just making 'g an anaphoric variable of 'mac that holds a memoized function that generates gensyms, so that g!x is (g 'x) as usual? ^_^
Then you can explicitly pass 'g to 'foodef. If that isn't enough, 'g could instead be a global function whose behavior changes during the dynamic scope of each macro expansion. Hmm, then you can't have more than one 'g at once, but when would you have that anyway? XD Something like Lathe's namespaces might use multiple instances of 'g, but those occasional cases can fend for themselves.
