The time estimation assumes that the implementation will be done in Xvnc (or perhaps Xvnc/vncconfig). It could fork off a process that executes scripts in a xconnect.d/xdisconnect.d directory. To me, this looks fairly easy to implement.
Hmmm... There is a notification system already in place for connect events (VncExtQueryConnectNotifyEvent).
Note that sessionreconnect.d already exists, but that is run as root on the master. The issue here is about running things in the session as the user.