Ok, now I am in contact with one guy that managed to run a working kernel that
he compiled himself from the source code that Motorola Hong Kong has published.
This finally confirms that the kernel (even though it was requested for E68)
works on a A780 without further modifications. On the other hand, I'm a bit
puzzled why it won't work here. To figure out where the problem is, I've asked
him to pass me the exact source tar-ball that he was using, plus detailed
information on his cross toolchain.
I've also started over again from a 'vanilla' Motorola kernel tree and will
give it another try. If this works, I'll re-try with the serial console, and if
that works, move on to the 2.6.x tree (which I'm planning to make public this
weekend, btw).
Meanwhile, I have confirmed that the bootloader is actually based on blob, and
thus also needs to be released under the GPL. This, in turn, should facilitate
the development of a GPL licensed host-side replacement of PST for flashing the
phones.
I'm a bit worried since I'm busy with many other things over the next couple of
weeks. But even while travelling, I'll have the full toolchain, sources, and
everything with me.