Another perhaps easier solution would be if the ECAM memo (or equivalent) would display the message '121.5 NOT MONITORED' in Amber.
EICAS already does it for frequencies given over CPDLC on the 777. Once the correct freq has been dialled in on Box 1 the EICAS message frequency changes from white text to green text.
Wouldn't take too much to add an alert similar to 'Pilot Response' linked to box 2.