How would you override Editor object dragging?

I'd like to make a script (or find one already), that will let me filter when game objects are dragged. Something like 'snap to grid' except I want to impose various constraints, like 'drag R distance in circle around This object' or 'only in the YZ plane' or whatnot.

I'm guessing I use the Editor.OnSceneGUI function, but the event types about dragging seem to be about drag/drop, seems like something different. I'd really just like to 'filter' the mouse position such that I can force the object to go to the nearest point my constraints would allow.

Any ideas?