So at least Zoom and Skype have disabled library validation to allow DAL plugins, for now, but we're still not able to provide those for Safari or FaceTime (at least on 10.14, which is what I checked), or Chrome, which is a pretty major holdout with this. @ieo my research indicates that a kext could be made that emulates a UVC (USB Video Class) device, which should be picked up by CoreMedia without a 3rd party DAL plugin. However, this seems like a daunting task, and is not a great proposition from either a stability or security perspective, either.

The INSTALL_ROOT setting is what worked for me.