It will be too slow to be of much use on most implementations. Perhaps it has utility as a reference implementation.
The most practical thing for Snow would be to mostly avoid stacking up dependencies on the details of regexps and to make sure that systems that natively offer Posix or Perl regexps can take advantage of that. Sad but true.
Yes, it probably is too slow.
Just so I understand, though, you mean that a Snow regexp package should present a standard interface that maps to whatever the native implementation is for the Snow host in use? Any preferences as to what that interface would look like? Going with Snow's least common denominator approach, what's the set of regexp features that all Snow hosts have in common? (Compiled vs. non-compiled regexps; submatches; replacement; etc.?)