Is there any technical reason that a video preamble could not be part of your software package? As a drill I set up FlDigi to do a video preamble, showing the letters 'VARA', it was a very quick transmission, and it seemed to me like it would sure clear up a lot of confusion, on the receiving end, about what mode is being used, especially as you advance your software into utilizing the CQ mode. A button that would send out a video preamble just once, on operator input, seems like it would be a natural fit for CQ mode.
While this would work, I think it would make more sense to have VarAC send a TXID when calling CQ, just like FLDIGI can be made to do. If Dave W1HKJ decided to include this mode in FLDIGI, then FLDIGI would automatically switch to VARA and even QSY to the CQing stations signal.
I don't remember seeing video IDs in FLDIGI for a long time.
Interesting idea. I will explore.