Mad,
I guess I cant directly address the question in point, but for arguments sake I'll add my take, maybe reitterating my earlier post.
I'm in the automotive biz. I have seen many instances that relate to the question, although it automotive rather than tractors.
Someone earlier made the point that the dealer was the judge. This is true in the fact that the MFG cannot be there to make all judgements. If the dealer sees that you have made a "saftey modification" they must note it somewhere. This will release the dealer and mfg from liability as long as you sign off and acknowledge the fact.
In no way could the dealer let you hang by denying waranty on your drivetrain because you cut your seatbelts out. Any court in the country would laugh that out..... I'm not a lawyer, but I have seen and heard from folks that this has been tried...
By drilling the ROPS you assume the responsibility because you have modified the original design that was tested to be safe. They cannot guarantee that it is still safe, therfore, your on your own.
Not sure if this added anything. But if a dealer tells me that the entire waranty would be viod if I modified a saftey device, I would be looking for another dealer.