Marc, my gut feeling is that exceptions and threads would take you far.
Especially the pattern matching example can be dealt with thru an exception
mechanism:

 (with-handlers ((umatched? next-pattern))
   ... match pattern (raise (make-unmatched "...")) ...)

would work just fine, especially since all functions could be global or in
a letrec, whatever is faster.

-- Matthias

P.S. I like elephants with trunks, too :-)