Btw, what games would it be useful for?

I ask it because the tesselation that you can force is N-patch tesselation (ATI TruForm) which is basically a DX8 feature.
It's available in DX9 as well but I don't know of any game using it (TruForm was not a long supported hw feature), especially with real DX9 level features (shader model 2/3).
Because of that and performance reasons, I enable forcing it in dgVoodoo only to DX8-level vertex pipelines.

Your request is basically the 'adaptive tesselation' in DX9, but how the tesselation level depends on the distance from the camera is application driven, that's why you can't force it along with the tess level.

"Btw, what games would it be useful for?"

I would be using it on older dx9 games where the models are ps1-esque quality if depth is unlocked. The reason i mentioned depth based is because im only going to notice the ppl in the game that are focused on. i dont really care about the environment being tesselated. So i figure if the range from the center (user configured focused depth) could have the tesselation only, it would reduce the strain of having it on this way in regards to applying it to the full screen. New games dont have it because the models usually are already smooth and not pointy or archaeic like. So if no depth is detected, it would work like it does now but if depth is detected, it would work in the proposed way.