The easiest way how to select from more kernels is kexec i.e. having small kernel in flash that can load full kernels from mmc or anything else. I think kexec for ARM is part of mainline kernel since 2.6.21 but there are patches for older kernels working on Zaurus and other ARM based devices. Also search this forum for 'kexec' or 'LAB', it was already mentioned. I have it in my TODO list for long time but so far didn't do any actual work. I expect there may be some extra work specific to Nokia tablets with keeping the watchdog happy when kexec-ing different kernel and also with passing parameters from Nokia
bootloader to second kernel. Also kexec is highly architecture dependent so there may be still some work to do to have it working on OMAP architecture.

So the short answer is no, it is currently not possible you can have only one kernel and in must be in /dev/mtd2 partition.

However one related thing that actually works is loading kernel to device RAM and booting it over USB without flashing. This is supported by Nokia flasher (-l option) and is somewhat similar to TFTP (i.e. not very practical for normal multiboot usage but very handy for testing experimental kernels).