Are FR ear canal inserts available?

Yep, but not necessary, And they dont have a rating because there is not a testing standard. You are required to have adequate arc rating protection for the head so the ear plugs FR rating is not an issue.

Thanks Zog. While I didn't have any great concern over it, by extrapolating from 130.7(C)(12)(c)-Underlayers- it seemed like there might be, or eventually will be some requirement for it.

I would not be suprised it someday it becomes a requirement.

Would not using a non fire rated ear plug, under your face shield, create the same result as wearing a polyester tee shirt under your FR clothing?

Your FR clothing could become "contaminated" (fiber transfer) by the polyester T-shirt, and not offer the same protection after. The canal insert cannot contaminate something else.

Plus, your ear canal shields the insert from direct exposure to heat, in addition to any PPE you can wear for your face/head.

That's part of the reason why shells are not adequate for ear protection from an arc flash perspective, they are exposed to the heat and could melt around your ears. The other reason why they're not adequate is that the pressure wave can kick them out of place, leaving your eardrums without protection (from the sound wave).