There are OSC messages to load a specific clip or layer onto the preview monitor. (/composition/layers/X/clips/Y/select and /composition/layers/X/select both sent without arguments).
But is there a message to eject what is on the preview monitor?
Selecting a blank clip is what I'm doing at the moment, but while it's an effective work-around in the short-term for my personal usage, it's still a hack. I'm working towards maybe releasing a version of my custom controller software and having to document "leave at least one clip slot empty for the de-preview feature to work" leaks developer hacks onto all the users.
I'd be okay with selecting over OSC a clip or layer with zero or negative column/layer index resulting in the preview eject, but my tests show that Avenue - understandably so - discards the messages immediately for not having a valid destination.