If you are thinking about going this direction then might as well put a high powerfed external USB wifi card on the N8x0. search this forum for "packet injection" chats and you will find some people using the external wifi for hacking into secured wifi connections. They already have some type of driver working but please research this before taking any action.

Here are a few examples of using just a parablic dish to amplify the wifi strength without spending much money. Yes it actually works with the NIT too, just you have to hold it in the center of the dish.

http://www.usbwifi.orconhosting.net.nz/

the n800 doesn't have very much power available on its usb port, barely enough to drive a lower-power flash drive (it won't even drive my microSD card in a Sandisk USB card reader), so I doubt you'd get enough to run a usb wifi adaptor.

my guess is that if you're really dedicated, adding an external socket is viable. Someone added an external wifi antenna to their Zaurus 6000. Mind you, it was a pretty fugly hack!

ace gave the only realistic answer.

If you got a mesh of routers with hi gain extra sensitive antennas they could use the signal that IS reaching them better, so a parabola at both ends.

the aim is to focus the beam towards the target without wasting it in the other directions.
I've seen people use parabolic woks to do long range bluetooth snarfing and wifi connections.
If you don't have a wok or sat dish handy you could rig something up with a couple of pringles cans and enjoy a nutritious meal in the process

Define dubious.

You can use an external router (with its own power) which functions as AP (using WDS). With a good antenna you can reach quite some distance. But it might not be portable enough for your liking.

You the get something like Bakscii.

Alternate might be 3G phone + tethering, N810EWME, or the NIT to be released in 2009.