Just happened to be looking around on here, and for the record, I didn't have any code specifically for this, I just added it, and I used this name for the set: sets.precast.FC.Inspiration = {}
While technically it could fit well enough in either FC sets or Buff sets, I felt it more fitting in this case next to the normal precast set, as people are likely to set_combine off the base FC set for Inspiration, .buff sets are generally (though not always) usually focused on a single piece of gear or maybe two, that affect a buff, whereas in this case, with all the extra FC from Inspiration one might have, one might want to swap around their entire set.
@Dodik "buff" sets aren't necessarily things that are equipped "for the duration of the buff", they are in some cases, but each sets.buff.whatever follows it's own rules as buff-affecting gear is WILDLY varied, for example:
sets.buff['Burst Affinity'] - Blu nuke midcast
sets.buff['Chain Affinity'] - Blu physical midcast
sets.buff.Diffusion - Blu buff midcast
sets.buff.Doom - all times when doomed
sets.buff.Entrust - casting a indi spell on another player
sets.buff.Impetus - meleeing or using a crit ws
Perhaps I should've done it differently, perhaps sets.buff shouldn't exist at all and they should each be part of their respective aligned actions, but, would be an awkward thing to change now at this point.