The SRFI 204 sample implementation is coming along nicely! I've written something similar in my Schemepunk library, which defines a Chibi-like match that works in 7 Schemes. You can see the definition of match in syntax.sld, starting at line 296. My implementation is structured a bit differently; it constructs a cond expression, and has a macro for the predicate (match?) and body (match-body) of each clause. It's missing a lot of the more advanced features, but could still be useful for reference. (I think and patterns are broken right now, because of changes I made to chain for SRFI 197.)

I noticed that you were starting to implement match for Gerbil Scheme. I've already done that for my implementation, and it was by far the most difficult to implement. The problem is that the underscore is a reserved word in Gerbil, and this cannot be overridden. Gerbil also has inconsistent support for Oleg's symbol-distinguishing macro trick. I combined the solutions to all of these issues into the syntax-symbol-case macro (syntax.sld, line 165), which might be useful to you.

Kawa Scheme might also be difficult. I tried running the SRFI 204 sample implementation, and it failed because @ is reserved in Kawa; once I removed the @ clauses, it seemed to work. It might be a good idea to decide on an alternate symbol for record matching.

Or is record matching even a necessary feature? Record matching makes this SRFI dependent on SRFI 99, limiting its portability. And some Schemes, like Chicken, only allow reflecting on records if you explicitly import SRFI 99 or set -D debug.