I've also wondered about just a ROTS face on an OT head framework (bald skull, short neck). I've heard of some people having problems wearing one of the current ROTS offerings. One implementation makes it difficult to fit the neck under the armor. Another was difficult to wear but it might be because the wearer had a really thick neck!
I've seldom heard of anyone having a problem wearing an OT-style head. The only complaint is that it's too big for some, so they opt for the Don Post Standard.
There are some mods beyond just the computerized mirror-flip that LFL did to the ROTS Vader. The squarish tear ducts, the whiskers' distinct shape, and the circular-like roundness of the nose and the specific shape of the silver colored inside-nose.
The bridge of the nose is also shaped differently and is atypical of the OT look, so I'm saving that towards the end.
Also the tusk tubes are -- when seen from above -- straight, whereas the tusk tubes on the OT are curved. I could be wrong; I may have to look more carefully, but they strike me as straight.
Another thing that's interesting is that if Vader is looking down, on an OT you can see the "V" shape of the lower tusk tubes as wider than the top ones. On the ROTS, the upper tusk tubes obscures the lower tusk tubes more, so the face looks more straight on the sides. Hope that makes sense.
I have a Portumac DP DLX recast. Aside from the fact that the recast suffers a slight loss of detail (moreso around the eyes, less with the cheeks) the flaws of this particular casting (of course, inherited from the mask Porty cast from, so it's not Porty's fault) are quite blaring to me. If I do an OT that's more symmetrical, I'd probably try just modifying this DP recast and see where that takes me. Would I want to make it perfectly symmetrical? Not sure.
In the OT masks, the teeth are actually off center, and the right side of the mouth is longer than the left, even if there is no jaw droop to the right. However, it's not impossible to fix this....