I'm not sure this will do what you want it to, since listers may open both before and after that point.
I probably don't understand what you need to avoid using a flag/event, but using OnOpenLister (which scripts can use to be notified both before and after the initial folder tabs are read) and a variable set on the lister may be a good way to do things, if the aim is to track changes to listers but only after they've opened.