AFAIK there's no way you'll get a response from the HSYNC and VSYNC signals, at least without modifying the video card's timing to match that of the Classic. Even then I would doubt it.

You may need to make up a circuit to feed the sync signals elsewhere into the analog board. I'm reasonably sure I've seen such circuit schematics around on the web. You'll need to make up something to adjust/invert the voltage levels too by the sound of it.

http://www.epanorama.net/links/videocircuits.html#syncconv

It sounds like it's time to grab a copy of the service manual for your donor Mac, and perhaps the Larry Pina book too. The "ugly Mac" project from that book (classic motherboard in a box with external video) is documented somewhere on the web as well, and may have valuable hints.

What Mac are you using?

Also useful:

CRT interchangeability

SE/30 repair - helpful if you're using a 128, 512, Plus, SE or SE/30

VGA to RGB includes a section on VGA to TTL