@Akrobah I do not know your Xivo PBX dialplan . You can set and access Queue variables in dial plan something like ..
exten => n,Set(thisQueue=${GLOBAL(QUEUE_${EXTEN})})
Mixmonitor monitor application funcion is
MixMonitor(filename.extension,[options,[command]]) .
So access the asterisk standard varaibles like extension and your custom varaible and prepare file name to pass it toMixMonitor. Also look at 'g' and 'h' operator of dialplan . 'h' operator allows you to do some activity on call after it completed , in your case renaming file .
You can send Mixmonitor part of dialplan and let me see if i can give you some hint .