It's a bit strange that I still have so much difficulty running my own kernel on the phone. As it appears, there are some subtle hardware (or bootloader?) version differences that made me struggle for so long.
Two out of my three A780, and my one E680 don't boot any self-compiled kernels but rather just crash. The third A780 however boots them just fine.
Obviously, as Murphy's law indicates, the phone it works on is my 'production' phone, i.e. the one I use for my day-by-day phone needs.
We really have to get to the bottom of what's going on here. Also, if there really are differences, then Motorola has to publish the kernel source for both versions under the obligations of the GPL. So far they have only released a single version.