Alternative version:

1/ There are some older Apple monitors that will do both 512x384 and VGA. Dig up one of those as a test bed, and make/find a reverse PC to Mac monitor converter.

2/ Sync that monitor and your video at the Classic scan rates and 640x480.

If that works, you *might* be able to scavenge the video amp from that and hook it up to the Classic CRT. Maybe.

I make no guarantees.