Hi Rochabian,
Nice to see someone else fighting with the drivers, not very common these days. Obviously we all want to use HD 5xxx, HD 6xxx and newer cards, and the fact we're not doing it already is that it is not possible with current techniques. Using EDID overrides is not going to help because apart from the few timings you can use the problem is the drivers won't accept all what we'd like in there (low resolutions, etc.) and even more important, the timings you put in there are read-only, and what we need is to obtain custom timings on demand as we emulate games.
This started to happen with HD 5xxx cards. Interestingly you can use the same driver binary with an HD 4xxx card and it will work fine. So it looks like a different path is taken inside the driver for newer models that bypasses our custom timings. It might be possible to patch the drivers to revert this situation, but it might simply not work. Of course if I knew how to do this it would be done already. It's not a problem of DVI or VGA outputs, it's general.
A different approach would be to insert the modes inside the driver's binary, as default modes. But again, this would make the modes read-only, which is not interesting at all.
Maybe a magic combination of registry keys would make it happen, but I hightly doubt it because I've tried dozens of combinations without success.