Multiboot can work only with kernels which boot initially from N900's Nand (rootfs).
Maemo and Nitroid are the only kernels which do it.

So the problem is not to make a config file for multiboot but it is to rebuild/extend multiboot to support booting process without rootfs.

And this is not a simple task.
So the real answer to a real multiboot system will be u-boot when the remaining problems on N900's version will be fixed.