Ok, well I think I've got a UI that will work for the filter mechansm - when you hold down Ctrl and click on the selection indicator a little filter icon will appear and any selection dot will be showed shifted a bit to the left of it:



Now it's a matter of making the filter actually work - that's going to involve modifying a number of different selection methods.

One thing I'm not entirely certain of is that if filtering should only only constrain selection actions only and not have any impact on deselection actions...

- Michael