I was going to make a filter for the "Hot Network Questions" by excluding some sites from it. I noticed this answer which talks about "negative filtering" but does not explain how one can do it or if it is possible. There does not seem to be a way to do this, though. On the filter page, my only options are:

So how can I do "All sites except [list]"?

Edit
It would also be nice if this could include similar functionality with the meta checkbox. Something like "All main & meta sites except..."