The problem here is that the FieldStorage implementation of mod_python behaves differently from the implementation in cgi.py. While the latter adds query-string parameters after POST body parameters, and does so only if the the parameter isn't already in the body, the mod_python implementation adds query string parameters first and unconditionally.
While the mod_python implementation is IMHO correct, cgi.py is the standard to follow. I'll attach a patch that modifies this behavior to make it compatible.
I believe this is preferrabe to hacking around the problem in the individual modules, because it decreases the probability of things failing in one frontend while working in another.
