To answer your question directly, either welding or drilling is ok. I drilled and tapped my ROPS to attach work lights (tubing is about 3/16" thick).
I have also gotten my ROPS modified - had it shortened it a couple inches so it would clear 90" door heads - this involved pretty extensive cutting and re-welding so the fender flange spacing was preserved. With some material addition and good welding prep and technique, the ROPS is stronger now than it was before.
On my Kubota tractors the ROPS tubing material is not hardened - it cuts and welds like a regular low-carbon steel, with no unusual sensitivity to welding heat. The material is steel tubing with decent strength properties, but not exotic.
From some experience I have in stress analysis, drilling a 3/8" diameter hole in the middle of any flat surface of a ROPS would have no measureable effect on its strength.
Regarding liability for mods to a ROPS, life's too short to worry about it. If you make a home-grown folding one, you can expect some legal hamsters to yammer if there's an incident where it collapses - but they will anyway. The real function of a ROPS is to keep the tractor off you in a rollover. They are very strong and not made of glass - welding and drilling within reason is ok.