>I guess a solution where also old binaries can be moved to other cores than the boot core is not possible.

No, of course it is.
But scheduling across multiple cores, its a bit more complicated than just being able to run code on more than one core.

And yes, I'd be pretty surprised if a few of the MorphOS developers hadn't toyed around with using the additional cores.
After all, MorphOS as it exist right now would not even have to be aware of the other cores' operations.