>
> (setq mac-command-modifier 'alt)
>
> (global-set-key (kbd "A-c")  'info)
> (global-set-key (kbd "A-z")  'info)
>
> Then A-z runs info but A-c just inserts ¢
>

That's because iso-transl uses key-translation-map, which happens before 
the key is searched in the global-map.

But what do you want to do exactly?  As I said earlier,

(setq mac-command-modifier 'meta)
(setq mac-right-command-modifier 'alt)

should I think give you what you want: a meta key on the left of the space 
bar, an alt key on the right of the space bar.