With each release we are reducing the hard-coded dependencies to OpenVAS, but it is not yet in a plug-and-play state. Eventually we will reach that state but it is not a dominating priority.
The recent step of dropping OTP in favor of OSP was a big one, though.
I never tried it, but I assume you need hard overwriting of the defaults to get another OpenVAS as default scanner. Or you simply let your other OpenVAS serve the file socket that is pre-configured. As long as it can serve the same OSP, gvmd should not notice a difference.