I just had a similar situation - also nothing backed up - well on the emmc... after being near a heart attack I tried to think logical and searched around...

what caused my reboot loop (sounds exactly the same as yours):
nitdroid installer failed - tried sandisk mmc specific kernel (see nitdroid forum) - rebootet using multiboot --> reboot loop (multiboot menu didn't show up anymore)

this worked for me:
pull battery out
plug in usb
issue the following command on the pc (as root/with sudo):
./flasher-3.5 -F RX-51_2009SE_10.2010.19-1_PR_COMBINED_MR0_ARM.bin --flash-only=kernel -f -R
press u on the device and while pressing insert the battery

in my case immediately after flasher showed its completion status I had the multiboot menu up and I could select the kernel

everything still there (contacts, settings, ...) just exchanged the kernel

(beforehand I was running the kernel-power)