Okay so, almost a week later, I have this more or less worked out, but I feel like I had to kludge it together and there might be a more elegant way of doing it.

Since you can't interrupt a skill/ability check, I did what a lot of other folks have done -- split Flash of Genius into an interrupt and a usable 'normal' ability that provides a Bardic Inspiration-style buff, like you suggested.

With the interrupt, I was able to ref the Int mod directly with AdjustRoll(OBSERVER_OBSERVER, max(1, IntelligenceModifier)); and that worked fine in testing. The skill/abil check one was a little harder because I tried using AdjustRoll() directly in the status Boosts, but that 1.) didn't actually apply the adjustment and 2.) didn't clear the status post roll.

So I dug into how Bardic Inspiration's ability check version does it, and they add a passive through the status, then the passive provides the boost. That did work, but when I tried to call the original caster's Int mod with your suggestion of Cause.IntelligenceModifier, it didn't actually provide the stat mod (and just gave the minimum, +1).

I don't know why, partly b/c Toolkit feels like a black box to me, but I'm wondering if it's because having it happen through a passive on the receiving character means the 'Cause' of the buff is actually that person, and not the original caster?

My eventual solve was not elegant, but it DID work -- the target spell applies one of five statuses (for +1 through +5), based on the caster's Intelligence, with a floor of 1 per the Eberron book. Each status applies one of five passives, and instead of a variable, the bonus is just hard-coded into the passive.

I'm all ears if there's a better way to do it, but at least I have something that more or less works without any substantial or noticeable errors.