Have you ever tried to use the 3 point UCS toolbar button?

Works pretty well to establish the origin point, X axis direction, and the Y axis direction with three mouse picks.  The command is actually a hidden command line option that is not shown on the command line when you use the UCS command.  All that is shown after typing in UCS is:

Current ucs name: *NO NAME*
Enter an option [New/Move/orthoGraphic/Prev/Restore/Save/Del/Apply/?/World]
<World>:

There is no option listing for 3, but if you type it in, the command acts exactly the same as the toolbar button.  The toolbar button actually has a macro listed for the action of the button that reads: ^C^C_ucs _3.

Sometimes the simple things in life are hidden from view.