Thanks for the feedback, everyone.
Ok, it looks like I can set a corner, for instance, by using D followed by -<x/2>, -<y/2> in either the DeltaX & DeltaY fields, or in relative mode in the Shift-tab fields. Still, I was a little surprised that there wasn't a way just to set the reference to the corner (unless there was something to snap-to in the corner).
Maybe I should back up and ask a simpler question, which must come up all the time. If you have an object, or collection of objects that don't necessarily have a corner to snap to, and you want to park it snug in the lower right hand of a box (snug against the bottom and side of the box) ... what is your standard approach?
Earlier I did this for myself by adding in some temporary lines, moving the reference point, moving the object, and then deleting the extra lines. But I figure there must be a more intelligent way to do this, and it must come up all the time.