How do I tell OS X to never include certain folders in Recent Places on Open/Save dialogs?
You cannot do it on a per directory basis. What you can do is:


Clear the current list with the following command in Terminal:

defaults delete -g NSNavRecentPlaces

Then repeat it each time you want the list to be cleared.
Disable saving the Recent Places history completely with:

defaults write -g NSNavRecentPlacesLimit -int 0