That covers the script's output, you're just missing the input.
How would information from the URL get to the script? For example,
in x-exec: you can say "x-exec://program/some/path" and "program"
will get "/some/path" as PATH_INFO. If it wants itself to be
run with "/foo/bar" as PATH_INFO it outputs <A HREF=x-exec:/foo/bar>.
How does the similar event happen with a the MIME type? The
best I can see is that the URL has to bounce off an HTTP server
which outputs the appropriate HTTP/1.0 object with the
application/x-csh-exec output type or somesuch.
In other words, how does the script reference itself in an anchor?
If you can solve that, then I don't think there's any problem
with other stuff like different methods (POST) and extra data
on stdin (for application/x-www-form-urlencoded or whatever).