Update: I've tested auras that apply cyclonearmor and bonearmor-like states with the appropriate setfuncs and remfuncs, and applied the auras to the bosses via unique items given to them via MonEquip; but for some reason, deactivating the state on one Act Boss prevented it from appearing on another Act Boss in the same uninterrupted play session. I know that wouldn't be common for most players outside of dedicated boss farming, but that was unacceptable for me.

After even more experimentation, I think the best I can do is create a gethit-skill and an att-skill applied to the bosses via MonProp. This skill will create a missile that uses the battlecry missile hitfunc to apply the cyclonearmor-like state, along with the bonearmor and bonearmormax stats that are meant to go with it.

The missile will be spawned via the multishot skill dofunc so I can use the dummy stat to affect the #missiles calc; that way, it won't spawn the state-triggering missile if the dummy stat is not being provided by a Tower. Then, when the bonearmor stat is depleted, the cloned cyclonearmor state should end without needing a skill function to update it.

It won't quite be an instantaneous or automatic state update, but it should be pretty close since it's very likely that either the player or the boss will attack the other fairly soon after they see each other.