How sensitive are the DVI sync signals?
Contrary to the commenters I doubt that monitors would have trouble supporting the standard 640x480 VGA resolution, as the DVI standard actually mandates support for a so-called "low pixel format" which is 640 × 480 at 60 Hz. Monitors with no analog input (i.e. no VGA input) will probably be equipped with a DVI-D interface (which doesn't accept a DVI-I nor DVI-A connector). 

My guess is that since pin 8 is used by digital inputs as well, you could try sending an analog vertical synchronization signal at that pin (its timing should probably match to the timing of the digital sync signals though).

The other thing which needs to be watched out for is the minimum clock frequency of 25.175 MHz mandated by the DVI standard. Anything below that will surely be rejected by the monitor.