Pete Zaitcev alerted me to the presence of kpartx in device-mapper-multipath. As kpartx is more generally useful for with MD, as well as hard disk images used by UML, QEMU, etc., it would be better if it was part of the base device-mapper package.
I'm intending to do that when I find the time, but the logistics are not simple.
Let me know what I can do to ease the move.
Peter Jones dealt with this by extracting it as a subpackage at build time.