I’d want to check WoR Supplemental again for the dates. There was a blurb in there about the Clans using Society gear and they agreed to use the ‘Pariah’ but not Cepheleus (sp?) because it’s not a true combat mech, and the Osteon was a maybe because of the ingrained Nova CEWS. But I don’t know if that predates their destruction.
The Grand Council voted on this in 3081, FYI, and only the Septicemia wasn't outlawed from use. All the Clans also shunned the Boggart, so there's no reason to think the Spirits would have adopted it. It does however note that the Clans' severe military depletion did allow them to use what they were able to salvage off the battlefield (this was specifically regarding ProtoMechs), but this was before the WoR Supplemental, which is where continuing usage was sorted out with the Grand Council.
So yes, there's a little bit of wiggle room there if one wanted to field these Protos with the Spirits in limited numbers, but given that the Spirits withdrew from the Homeworlds post-WoR and certainly wouldn't have fielded them before then (not with the Vipers purifying everyone's taints), the window seems kinda small to me (i.e., during the initial Cobra/Lion/Coyote seizures on Haven and Honor and their shortly-thereafter Annihilation by the Adders).
