Let me summarize my understanding for your installation:
INTERNET ------ OpenACS ------ Backend
You want to do all authentication on "OpenACS" and forward some requests to the "Backend" with authentication information.
When the reverse proxy is registered with a postauth filter as a indicated above for PUT requests, I would be really surprised, if it does not grab the PUT requests.
However, as authentication is implemented in OpenACS as a preauth filter, you have to make sure that this preauth filter is called also for PUT requests before the postauth filter, since stock OpenACS just registers the preauth filter for GET, HEAD, and POST requests (see e.g. packages/acs-tcl/tcl/request-processor-init.tcl.
The OpenACS error message about "auth_level" is not a valid option" hints this way.
