Shui, thanks I didn't know about the alt-click-drag feature, very useful in itself.
I think this same approach could work for enabling stretch mode - holding Alt while hovering/dragging over the root/end of the bone enables Stretch Mode - you could print "Stretch Mode" or something in the center-screen tooltip so people know what's going on, and change the little red Drag Length icon on the bone tip to an XY Translate icon.
As is, I can only see this working in Setup, it could get quite messy in Animate mode as you could get unpredictable interpolations on long bone chains, and would probably cause all sorts of problems when you start looking at putting IK constraints in.