I'll admit that, along with Kent Dybvig, I'm no fan of arity inspection,
for a jillion reasons.  I think it's ill-conceived.

But if it must happen, how about this:

(procedure-arity PROC) returns two values, first, the minimum number of
arguments, and second, whether additional arguments are permissible.
This exactly covers the possibilities for standard Scheme.  It is
trivial to implement given any of the common facilities out there.  It
is clear and simple and easy to specify.

Thomas