Screen space tool was what I was thinking, I was just going off what I thought of for the sequence of mouse gestures not an actual radius input value. I am sure it would be quicker to just do it in screen space rather than draw out a curve just to be a reference for the command.

Hot Key circle select
pick center LMB
drag radius LMB
select in or out RMB In or Out could be a visual arrow cue like you use for the image planes.

Now that I think about it the drag radius LMB step would just be a reference for another radius value that would determine inside or outside selection.