Syncro,
Yes, the liability is yours if you represent the ROPS as a structure that meets all federal and state requirements...
In your case, if you sell it, I would make sure that a bill of sale stated that it is a Ford 4000 ROPS. Any alteration to the structure that could cause injury would be on the part of the individual that made the changes, and not to the seller. And I am not an attorney either.
It has been talked about quite a bit in other posts and the general consensus is to call it a heavy duty light and canopy support if the equipment is designed and contracted out to a welder, fabricator, etc...
My first choice is to have one manufactured by the parent company (Yanmar, JD, Kubota, etc.). My second choice is to use my own design, which I have a mock up sitting in my shop. Here in lies my problem, and I think it is a big one, If I sell one of my tractors with the HD light-canopy AND seat belts than I have misrepresented the tractor as meeting all safety requirements.
It is a real Catch 22.
Good luck, Mark