If I understood correctly - you must disable 1 API call and this call is identical in all thrash drivers?
No, this patch a little more complicated.
It seems that thrash API according to fog was changed when NFS4 was released. So, NFS4/3dsetup/d3da.dll (and d3da.dll from the NFS4 DEMO) can work with fog in NFS3, but NFS4/d3da.dll can't. It needs investigation. It seems that API was slightly changed, because all control codes according to fog is same. Maybe latest thrash drivers need some additional init code for fog or so.
What's interesting, original d3da.dll from NFS3 also knows about fog and it uses same control codes. Maybe this support hadn't finished when NFS3 was released, so developers disabled this ability.
If you know any d3d thrash driver which displays fog in NFS3, tell me. You can test any thrash driver on Windows XP, but be noticed, that original NFS3 doesn't try to use fog in D3D mode, so use it as voodoo2. Or use latest beta of my patch, where is possible to enable fog for any thrash driver.