Perhaps you could temporarily install Reaper and register it in that?
There’s a possibility that the plug-in may still require “phone home to verify” for it to run in Audacity, which would probably be impossible in Audacity.
This really is up to the plug-in developer. And for example, if you go to the WAVES, Anteres, or Celemony websites they have a list of officially supported hosts. Audacity is rarely officially supported… Sometimes they’ll work anyway but I assume most Audacity users don’t buy plug-ins so it’s probably not worth the trouble of testing and debugging or dealing with Audacity support issues. One of those (I think Anteres) specifically says that their plug-ins do NOT support Audacity.
With free plug-ins it’s usually just hit-or-miss because there is no budget for testing & debugging with multiple DAWs.