I have had a go at modifying multiboot, to create boot.scr items for u-boot, attached is the very first rough version.

though it is named the same, it functions differently from the original, the difference will be explained :

this one works by listing files with .cmd extension on your sdcard, when you choose one, it will be converted to a u-boot script and called boot.scr, the next time you start the phone u-boot should read your boot.scr file and boot your OS of choice..

the extension .cmd is completely meaningless, this is just used to differentiate the files.

you are expected to create the uboot command files manually, you are also responsible for converting kernels to uImages, Initrds to uInitrds, you are also responsible for placing these in the correct places specified in the command files.
but this is done once, so it ain't a problem right ?

meaning, this is just a menu, nothing more !

besides listing the command files, the original view is also retained, however selecting an item with a kernel different from the one currently loaded will simply do nothing, well may be screw up the view a bit as I have loaded fbcon as I wanted a shell, you can disable it if you want.

the original view is needed when you want to start backup menu or nitdroid, meaning items that share maemo's root fs, or use it as their initfs.

the item that needs the kernel that you currently have loaded will be colored in green.

just a note about nitdroid :
first you have to create a command file that will load nitdroid's kernel but will use nolo supplied atags, this will enable multiboot to start, from there you should select the nitdroid item normally, so basically first time choose the .cmd file from uboot section, second time choose item file.

another note : please donot reboot, always switch off your n900 then start it again as u-boot fails to detect sdcards on reboot.

DISCLAIMER:
PLEASE BACKUP BEFORE TRYING THIS !!
THIS HAS BEEN TESTED ONLY ON MY DEVICE, AND IT IS PROBABLY DIFFERENTLY SETUP FROM YOURS, AND FAILURE TO BOOT IS HIGHLY PROBABLE.

to use, you are expected to have multiboot with backup menu support installed, then extract the attached zip file somewhere on MyDocs, cd to where you extracted it, then as root run the runAsRoot.sh script, MyDocs is not executable so use sh ./runAsRoot.sh

or just copy the files manually, noting the destination from the script, just please don't forget to chmod +x /sbin/multiboot

AGAIN PLEASE BACKUP FIRST !!!