Reporting after flight:
While boarding pass doesn't mention that the specific seat is "asiento +" or the large carry-on allowance, it did assign boarding "GRUPO 3".
It was mentioned at the gate that passengers with GRUPO 4 and 5 would likely have their large carry-ons put on hold (for free) and that passengers with GRUPO 6 (other BASIC fare passengers) weren't allowed a large carry-on.
I boarded with my large carry-on + purse without any problems! So now I do believe that, while it isn't published as clearly as we would like, the benefit does work.