I'm more worried about the compatibility of some direct3d games with the 3dfx hardware itself, rather than kekko implementation - On top of my head, I remember having an hard time running the original Colin mc Rae Rally on HW acceleration with a voodoo card.
I suppose this could be fixed, but it could require straying away from the original hardware, and possibly some changes to the (open source by now, i believe?) 3dfx reference driver anyway.
For the Direct X 7 stuff - it was the time when most game were starting to require TnL support and extremely higher clocked CPUs, I doubt that dosbox can run them even on modern computers.
About the complexities of writing the driver - Well, I remember talking years ago with some co-workers whose job was writing windows drivers for soundcards and other multimedia devices, and yes, they didn't really describe that as a pleasurable experience. Probably the most difficult part of taking this approach. I suppose that there are some old windows DDKs floating on the net, but I pity whoever would take this job.
About the Gallium3D driver - isn't that just a passtrough? That wouldn't be very helpful, given that most fixed-pipeline functions and features have been deprecated on modern videocards.
Anyway, too many "woulda", "shoulda" and "I believe" here - I'm going to let the actual developers do the talking 😀