My theory here, which I'm hoping to have confirmed or dismissed - is it possible my narrowband O2s are getting fouled with raw fuel causing them to read ridiculously lean here?
no way

false lean readings are usually exhaust leaks or misfires (things that pump air that hasn't been involved in combustion into the exhaust)

a tiny spot of lean right after stabbing the throttle is kind of normal. you're introducing a lot of air really quickly. this is what 'pump shot' like routines are for, since they follow the tps instead of waiting until your more accurate but slower fueling methods catch up. it's also what 'burst knock' is for, since a momentary rush of air like that can cause a knock or two. so a blip of lean o2 readings never concerns me if it recovers quickly

i'm a bit concerned by that, though, since you're already well over 6000 rpm, you'd figure it would have caught up already. you might want to confirm you aren't getting pressure drop or experiencing some other problem.

or just add fuel and see if that helps?