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.