Another perhaps easier solution would be if the ECAM memo (or equivalent) would display the message '121.5 NOT MONITORED' in Amber.
This assumes that the software of the ACPs (audio control panels) would be capable of modification to detect if : the box 2 audio buttons were selected, and/or that 121.5 was dialled up in the active channel.
Airbus and Boeing et al can have this idea for free in the interests of better safety.