Take the u-boot.bin, pad it to 0x40000 bytes. Turn the normal zImage into a uImage then append to padded u-boot.bin. The total size of the resulting image must be less than 2MB, which is easily possible using the PR1.2 kernel. Finally flash the new combined image to N900 like a regular kernel. You can use builder.py (below) to make the combined image.
File "builder.py", line 13 elif len(kernel) > 0x1c0000 ^ SyntaxError: invalid syntax