Rewriting all symbols is what the CL package system does. Of course, the CL package system does not really rewrite symbols after parsing, it chooses different symbols for the same string in different packages when parsing.
A true module system just rewrites all identifiers that are globally referenced, without touching local identifiers and quoted symbols.
Why not local identifiers? It won't hurt anyway, since:
(fn (x) x) == (fn (y) y)
Quoted symbols are more problematic. The problem is that you can't determine how macros will bash quoted symbols. For example, (tag (x) ...) implicitly creates 'x. And prior to replacement, you can't be sure if the 'tag here is pg:arc1's tag or some other module's 'tag macro.
An alternative is to make the (pr ...) versions of similarly-named symbols in different modules the same; this probably involves hacking (coerce ... 'string) to cut out the module name for symbols.
Hmm, probably means we need a separate macro namespace. Heck, stuff like (let obj (table) (obj 1)) doesn't do as you expect, because of the mixing of macros in the same namespace as functions and values.