Cannot set an env var (GSSPROXY_SOCKET) that mod_auth_gssapi will use

I have multiple web applications running with different keytabs and gssproxy. I use [gssproxy's sockets](https://github.com/gssapi/gssproxy/blob/main/docs/README.md#aside-using-a-custom-socket) to differentiate between them. For that, I need to pass the `GSSPROXY_SOCKET` environment variable to `mod_auth_gssapi`, and I can't find a way to do it. I've tried:

- setting one in `/etc/systemd/system/httpd.service.d/`, that works fine but I can't differentiate between the web apps running in the same apache
- using `SetEnv`, which is [documented not to work](https://httpd.apache.org/docs/2.4/en/mod/mod_env.html#setenv), and indeed it doesn't
- using `SetEnvIf`, it doesn't work either (I tried `SetEnvIf Host app.example.com GSSPROXY_SOCKET=/var/lib/gssproxy/app.sock`)
- using `RewriteRule`, and it doesn't work either. I tried `RewriteCond %{HTTP_HOST} app.example.com \n RewriteRule .* - [E=GSSPROXY_SOCKET:/var/lib/gssproxy/app.sock]`. I did check with increased logging that the rule was matching on the request.

Is there a way to pass an environment variable to mod_auth_gssapi, that is dependent on the virtualhost or even the request path? If so, it would be great if it could be added to the docs (here and/or in [gssproxy's Apache doc](https://github.com/gssapi/gssproxy/blob/main/docs/Apache.md)). Thanks!