I too would really like this as it would mean that smaller guilds could also use it without having to make GC changes while others are online.
I've looked into it a little and it seems that the LUA file that GroupCalander stores it's data in could be used but some of the data in there is stored as account/character specific.
I have no idea how to do it for GEM cos I don't use that.
I have WoWRoster integrated with my DF website and have managed to get Roster to record which user account on the website "owns" each character based on the user that uploaded the data... so it would seem from that it would be possible... if messy... to create an extra little page that could take the event calander data and match it up to the relevant user and thus their toons etc.. I'll look into it some more when I get chance but if anyone already has a solution they would like to share please do