I've thought about it, and decided not to do it. Reason is that it would take too much explaining and it would only work for certain configurations.
If you have a DVI-connected screen, then you'd expect to get the Cybervision signal through that channel as well. This would require me to make a A-D conversion for the Cybervision signal on Indivision AGA - that's very expensive and it would still be the same (if not worse) quality.
Switching the signal would only work for analog, but then the benefit of DVI is gone. However, if you have a DVI flatscreen, you have two inputs anyway (one analog VGA and a digital DVI). What's so bad about selecting the source on the monitor?
What we could do (and this is actually the plan for the next updates of all Indivision cores) is that we provide a common API for all Indivision models that lets external software switch off the Sync signals, so an external monitor switch or the monitor itself can do an automatic selection. I'm assuming that DVI takes precedence over VGA on any modern flat screen, so this will provide the functionality that you're looking for.