To switch back to the official kernel-power then run as root "apt-get install kernel-power=2.6.28-maemo46 kernel-power-modules=2.6.28-maemo46 kernel-power-headers=2.6.28-maemo46 kernel-power-flasher=2.6.28-maemo46", you can change the kernel-power version to whichever you wish to install (there is quite a few in the repositories, presumably all the version that have been released), this worked for me, but I am typing this from memory so may have made a mistake and I don't know how it interacts with multiboot because I don't use that.

As it happens the reason I wanted to downgrade was because of a bug with bluetooth audio that happened to be introduced in kernel-power v46 (it turned out not to be specific to the changes made in this version), and I didn't realise because I upgraded to this kernel from kernel-power v45, so slightly off-topic, but does anyone know where I should report that bug.