According to a Mexican fan I talked to, he knows to persons, who have the complete concert in video in their archive, which was transmitted on channel 52 from "Nuevo Leon" which is the state, (Monterrey is the capital city of Nuevo Leon) and they only showed some parts because of a cultural festival, but both of them supposedely have the concert.... There's a convention here in Mexico, on October, I didn't know of it, I'll go and I'll be asking about it, hope I can get something and if I do share it here....
Are you sure you're not getting confused with Puebla? The only known clips of Monterrey on youtube are from a amateur camera filmed by the local university but I thought it was Puebla that was believed to have had a professional filming which was once shown at a convention ( and legend has it that the guy who showed it disappeared without a trace and would not copy it for anyone anyway)
The Queen live in Monterrey 1981 complete video exist (I saw it) but the guy who has it never will share a copy, why? I don't fucking know, I only had the "privilege" to see the entire video at the house of that guy and that's it.
I hope someday it will be available to all of us, i'ts a marvelous amateur video recording.