I doubt it's that unusual to see a rainbow in that configuration when you're at that kind of altitude. Since a rainbow results from the scattering of sunlight through liquid water drops and since it's common for one to be within the clouds at nearly 12,000 ft, and given the sun angle, I would guess that happens more often than you may think. It's just that fewer people are up there and for less time, so it's less likely that someone would witness it and document it.
Still cool to see, though.