quoted

I have a cuImage kernel in order to support legacy u-boot and a
ramdisk image. Kernel boots fine if these two images are separate and
"bootm $kernel $ramdisk" is used. But I can not make it to work using
a single multi image that contains the kernel and ramdisk images. Is
it even technically possible to boot a multi-image with cuboot
wrapper?

Try the following steps:
------
1. cp <your ramdisk.gz> arch/powerpc/boot/ramdisk.image.gz
2. make cuImage.initrd.<your target>

You can get one Image, cuImage.initrd.<your target>, including kernel and ramdisk.

A follow up question. With this method, the total image size
(uncompressed) is limited to the 4MB (the link address of the boot
wrapper)?

Thanks,
-Shawn.