I mean that you can use flasher-3.5 for booting zImage. flasher-3.5 has option -l (instead -f) for loading zImage instead flashing. So you can download fiasco image, unpack it and boot (without flashing) stock kernel:
$ sudo flasher-3.5 -F <fiaco_image> -u # this will unpack fiasco image
$ sudo flasher-3.5 -k zImage -l -b # this will send kernel zImage to N900 RAM and boot it

Of course you can flash stock kernel to N900, remove u-boot, but flasher-3.5 has support for booting other kernels without need to flash it.