My question: is Mexican oregano interchangeable with the usual dried
stuff
(most of which ends up on pizza in my house, anyway)?

I've never actually tasted it- at least not to my knowledge. But from what I
have heard Mexican oregano is stronger and less sweet than its Mediterranean
counterpart. There's a good chance that my tongue - which has been
repeatedly bludgeoned by too many habaneros- would never tell the
difference.