Although it is possible to assign a new value to "req.filename", it is not possible to update "req.finfo" based on the new filename.
Suggest that if "req.filename" is assigned a new value, that apr_stat() be automatically called to update "req.finfo". Ie., internally mod_python would do something like:
apr_stat(&r->finfo, r->filename, APR_FINFO_MIN, r->pool);
I believe that mod_perl supports a similar feature, but would need to confirm this.
Related to "req.filename", the "req.canonical_filename" should also be writable as when changing "req.filename" the latter should also by rights be updated as well.