you could perhaps even have it make some sort of will or concentration check to NOT be distracted by the person on its back. You'd probably have to make it a harder check to not be distracted so much by it that it had an AC loss to the surrounding party members, but still remain "distracted" enough to not lose as much AC to the person on its back.

For that matter, it'd probably lose a substantial amount of AC to the person on its back. Provided they can stay on well enough to attack, they're in a very good position to do serious damage.

Hmm come to think of it, I can see why his DM refused to field the action you could really make it QUITE complicated without trying very hard... Still, creativity like this should never be squashed.