Lots of them are bolt on.
I had a Treg (override)which was bolt on and now a Qld caravan Components (like an Orac but no brake override) which bolts on.
Trouble is the bolt pattern is not the same and I had to drill new holes.
I think the bolt on ones have to be certifed as such from new, and the flange is usually bigger than the ORAC weld on.
If you can sort out the bolt pattern issue no problem, particularly if unbraked. Seems like a lot of hassle for not a big problem though IMHO.
Regards Philip A