Excuse me... But do you seriously intend to boot Symbian with Multiboot?
You do know what kind of OS you're dealing with, right?
It's not Linux. It's Symbian. There is no initrd, no modules, but there's a kernel image. That image is compatible with U-Boot, which we are fortunate to have.
Symbian is a one-of-a-kind OS. It's not based on anything else, and nothing is based off it. And in addition... I want to ask: do you have any experience or knowledge about Symbian whatsoever? I think you should read up on it before trying to port it, so that you can understand how it works. Otherwise it will be like arriving at mars for the first time.

I have belief in that U-Boot will work, as it does the exact correct thing for Symbian: use a kernel image, and then boot off a second partition on the card.