I already managed to do it for D3D8 by combining dgVoodoo with Special K, but am having a lot of trouble getting that combo to work with D3D7 and older. Those games simply refuse to launch. That was a bad assumption on my part. The problematic game is Thief 1 (It does more-or-less work with only dgVoodoo). Tried another game, Spider-man 2001, and the combo does work.
Please reconsider if you could make vsync disabling a general dgVoodoo feature. It's rather niche, yes, but it does have its uses. One such case is VRR displays. Usually disabling vsync and capping the fps gives a better, smoother experience.
If not, as an alternative, are you ok with trying to figure out why dgVoodoo+Special K is so problematic for pre-D3D8? I asked on the Special K discord server, but haven't received a response from the devs; probably are not interested.
EDIT:
I'd have to test more to have a clearer picture, but after testing a few, it seems the interoperability is hit-and-miss and rather problematic for pre-D3D8 games.
EDIT2:
If none of the above, sure, I can use one-off exes with vsync disabled.