As far as implicit gensyms go, it is certainly nicer than having to explicitly use w/uniq. I can see how it can get rather tedious though, and starts to make your elegant code look a lot like Perl.
Have you considered moving the emphasis away from macros and toward fexprs instead? I suspect if you primarily used fexprs, then the problem with implicit gensyms would go away. Of course, then you end up with new problems... like this:
(fexpr foo (x)
(let x 5
(foo x)) -> would this return x or 5?
I attempted to add fexprs to ar, and in doing so had to face this problem. I "solved" it by making eval implicitly evaluate in the outer (caller) environment, ignoring the fexpr's environment. You can, however, explicitly evaluate in the current environment by passing a second argument to eval.