Yep. I got the AA record locator from CheckMyTrip, and that provides access to the AA leg under the AA code (other legs are also shown in the AA system, though, as long as I use the AA record locator to open the booking). The AA leg under AY codeshare seating is however to no avail in the system, nor do either airline's call center persons seem to have access to it.

Well, I have to hope the system automatically assigns me a decent seat. As the AA-leg under AA-code got an automatically assigned seat in regular Y seating, I am not too hopeful on getting automatically assigned to a MCE seat on the codeshare leg either.