Originally Posted by anabolism

Absolutely the OP should look at alternate flights and routes they find acceptable or preferable and that have F space. Being routed LHR-LAX-MIA all in F would be much ore enjoyable than LHR-MIA, in my personal opinion.

Originally Posted by ISTFlyer

CMN-MAD-LHR-LAX-MIA is not permitted by the MPM limit of the fare and during rebookings agents need to be sure that the MPM is not exceeded.

Although, some agents would forget to look at the MPM thus the passenger might be lucky.