No, the rule, un der the "simple modifications" category is this:
The welding, chrome plating, heating or bending of
axles, suspension or steering components, as a method
of repair or alteration, is not permitted.
Verbatim from HERE
To me that says you can't use weld on coilovers. Nothing specific about bolt in coilovers but if you met the bump stop and other clearance requirements (height, shock stroke, captive springs) then you'll be less likely to get nailed.