Have you registered the callback also for POST and PUT? The revproxy callbacks work the same way for all kind of HTTP methods. I am not sure, what problem you have. e.g. rewriting the BODY of the request is not want you want to do in the PUT case.
There are some servers, who have problems accepting query variables and form data at the same time (this was a problem with AOLserver). It this the problem you are facing?
Concerning rewriting the form body: everything is possible, but i would not recommend it, since you have to care for the following cases:
a) format application/x-www-form-urlencoded vs. multipart/form-data
b) content spooled to file or memory
c) updating headers (length etc.), handling transport encoding (compressed), ....
so passing the value as a query parameter is easier. If, for whatever reasons this is not possible, consider adding an extra header field.
