I accidentally deleted some files from my N800 which is causing it to boot up to a white screen (it is working OK, but everything except for some text is white on white, rendering it pretty useless). Is there a way I can untar a tarball into the internal flash using the USB connection? I have no WiFi where I am, and the only place where I can get WiFi I cannot know the N800's IP address externally. Any help?

Problem is that I cannot set anything up on the N800 by using its screen, which means that I cannot connect to any new networks. I know I can flash it... it's just that I'm willing to wait a bit to see if I can find a better way!

Yes, that's what usb network recovery mode in latest version of bootmenu allows you to do. But it is too late now for installing this. Now only ready made initfs image specific to your device and firmware version would help you. Then you could flash such initfs via linux flasher and boot into recovery mode.

Hmmm, does anyone volunteer to do this image for me? I'd be grateful!

(I'm running the original Skype-enabled firmware with the latest kernel on an N800; but I don't think the kernel version would be such a problem.)

If you are sure you can find your way (i.e. know how to make usbnet working with your system, have ssh or telnet, can mount internal flash or mmc by hand when logged in via ssh, ...) than let me know.