Is it possible to configure Apache's mod_proxy to treat 503 errors as 200, and just forward them to the client directly instead of showing its own error page?
HTTP status code 503 (Service not available) means apache does not have any threads available to service the request, or there was an internal error causing the thread to malfunction.
This is an unrecoverable (network-level) condition.
Quoting the RFC:
Now, this seems to imply that the web server may answer a 503 condition however it sees fit; in the specific case of apache, however, the code is such that the request cannot be satisfied, and the request is not passed on to any code that can service it.
In plain terms: once a request has been handled (or failed to be handled), that's it.