It would be best if implemented as in Visio or Dia. (I think its called "smart shape")
The idea is simple. Attache a "snapping points" to a group. If you draw a line it would snap to this specific point. So you could define for a rectangle 8 snapping points (4 in the corner and another 4 in the middle of each edges). It would be really handy for diagramming especially if a box is not jusa box but a complex graphics with blur and image, and the snapping would become non-obvious. In that case you could just define some snapping points and the diagramming would be simple and easy and not time consuming.
Splendid! So we have a start of an interface proposal! The snapping points should be any node though. Maybe the nodes of a destination option can appear upon mouseover. Is it implemented as a separate tool or as an option in an existing one?
Just as aside, long ago Peter Moulder checked in some code which is supposed to allow objects to stick to guidelines and be moved when the guideline is moved. It required some changed deep in SPItem transform methods. I think by now this code is not functional, if it ever was.
Maybe it's a good start though! Is it still in there somewhere?