I recently attended a general aviation workshop with the ANSP here in Ireland. One of the topics covered was listening squawks (FMCs) and their underutilisation. Would it be possible to overlay the local listening squawk alongside the local FIS frequency? I've attached a screenshot as an example (FMC under frequency). 

Possible, sure. These are quite regional though, with many countries not using them at all, and other countries applying them somewhat inconsistently. We would probably want to see them become more uniform like FIS are before considering adding them for permanently display in virtual radar.