i think the reboot loop while on different kernel is caused by lack of kernel modules in /lib/modules. Imagine a situation:
- first N900 has kernel power 51r1
- second N900 has different kernel (cssu, or other version of kp... I think it would work if second N900 would have stock kernel)
you create a copy of rootfs and optfs of first N900, then you override rootfs and optfs of second N900 with it.
result: in /lib/modules you have only modules for stock and kp 51v1 kernels. If you try to boot e.g. kp 52 - oops, no modules found, reboot loop.

feel free to correct me if I'm wrong