The ROPS police are probably already circling in thier black helicopters; have you looked outside recently?
Generally, for liability, the manufacturer will say not to do this.
I grew up around Jeep's and racecars(1/4 mile oval). The first thing we ALWAYS did, was modify the ROPS(read roll bar) in the Jeep with a weld-in front cage kit. And, we welded cages, engine hoops, Petty bars, ect in to stock cars.
IMHO, I see no reason why you could not have brackets or tabs welded on your ROPS. They should be laid out well, and either welded properly, even if that means going to a certified welder or fab shop. I would not go overboard mounting all kinds of stuff and drilling lots of holes in it.
BC_Realtor noted that clamps are available; you may want to check out that route so if you ever want to move the extinguisher, or sell the tractor, you can just take it off.
I would note, that for my example, both weld on and bolt/clamp on cage kits are available now for Jeeps. Buddy of mine did one in his Rubicon. All the attachments to the existing rear roll bar were clamp on.
If mods to the ROPS were such an issue, then I would expect thousadns of lawsuits for Jeeps that rolled after cage kits were welded and/or bolted in. And, most FAB shops would be out of business, because they could not modify and/or build safety devices.
If you search TBN, there is a plethora of discussions about welding or drilling ROPS. And the ROPS police in thier black helicopters...