ninefinger,
This thread has turned out to be a lot more interesting than I expected! It never hurts to bring up an old subject for further review.
Wouldn't the liability issues only matter if the ROPS failed (e.g., collapsed)? If someone got injured or killed and the ROPS didn't collapse, would it matter if the ROPS had a work light welded to it (assuming the work light didn't injure the operator)?
I think the main reason for welded on brackets is convenience. Hose clamps and u-bolts/square-bolts work well, but they're a bit "fussy" to mess with compared to tightening up a couple of bolts in a fixed bracket.
I wouldn't drill a big hole in my ROPS, but if I wanted to screw something to it, I wouldn't be afraid to drill a 3/16ths hole in for a sheet metal screw. And I wouldn't be afraid to do some light welding to attach something to it. But I'm not recommending that others do this; make your decision and assume your own liability.