Following up on this, it does work, sort-of. The RG-11 works as expected, the Beaver Dome Status app correctly reflects the Safe/Not Safe status, but that status does not make it to the ASCOM Beaver SafetyMonitor. Status there is always not safe. NINA never sees the correct status so I can't use it there, but the Beaver Dome driver *will* do the right thing and close the shutter.
I've been in touch with the developer and apparently I'm the first to try this. It's not a documented feature and support for the SafetyMonitor was only added to the software in the last update. It's mostly there it seems and they're going to look at why it's not making it to the ASCOM interface properly.