Glad that you found a solution.
Note that there may till be some issues attempting to run the plugin in the background. It will still likely render a stereo view and attempt to submit it to SteamVR every frame.
The plugin also calls the IVRCompositor interface in many places. This interface is only designed to be called from the foreground scene application, so it might have consequences like messing with the frame timing and rendering of the scene app, and the frame timing and device poses of the background application will likely not be synchronized properly. Without the synchronization it will not vsync properly, so the app may run with an uncapped framerate.