So does anyone have any good ideas for adding an encounter with the Black Cowl BEFORE the end of the adventure? Has anyone done this in their game?

You can see that in my adventure thread, the Black Cowl made an appearance killing an underling before they could be captured (and because they accidentally/unwittingly tried to assassinate the Black Cowl).

I had 2 players "infiltrate" the BC organization in Averland.

They got to attend a meeting at a fancy hotel where the BC made an appearance and told his "loyal followers" how pleased he was about the consolidation of crime power on the docks. He passed out praise and some rewards.

This showed the PCs how many people were in on the take and that the BC is influential enough to hold a meeting at a gold tier establishment. However, the situation was a certain death if they attacked...

The players could have tried to talk with the BC (think speaking out at a town hall meeting) but they decided to keep quiet.