OK, this is what I have now. I think it's a pretty good solution.
On Tue, 28 Feb 2012 15:14:29 -0800
Joe Neeman <address@hidden
> Don't use ly:axis-group-interface::add-element, because stems don't
> implement the axis-group-interface. (Removing this will also remove
> the axes warning.) Instead, use ly:grob-set-parent!. You'll probably
> want to set both the X parent and the Y parent. Then the X and Y
> offsets of new-stem will be measured relative to stem (instead of
> relative to the whole system, which is the default).