EDIT 0: if you have not already done so, try to tftp flash an orginal netgear firmware first as @mk24 has suggested (the same way you would tftp flash an openwrt "openwrt*factory.img")
EDIT 1: see if you can find an "older" netgear firmware image (say from the openwrt version "18" timeframe last year) and see if that will flash, and run. If it does, then try to tftp an openwrt image over the older netgear image.
Before flashing a sysupgrade image to nand from an initramfs boot, I'm pretty sure from my own experience you must have first tftp flashed an openwrt factory image.
instructions to boot an initramfs image for r7800 here
after that I think @mk24 is suggesting you flash a "sysupgrade" openwrt image... (assuming the prior flashed version to nand was an openwrt image)
if the initramfs image does not have luci you'll have do something like:
scp openwrt*sysupgrade.bin root@openwrt.lan:/tmp
from your PC, and then from router
cd /tmp; sysupgrade -n openwrt*sysupgrade.bin
I've booted initramfs images on the r7500v2 but never tried the sysupgrade -n thing. good luck