I have wondered the same thing in the past although I guess it was just that softmods took those who would be inclined to play such games out of the picture.
A note though: the dumps might have been done but most programmable chips will feature some basic encryption meaning you can not "clone" a chip (the dump is essentially useless). I am not sure how the original programmers would have played it but the rest of the post will assume it is not encrypted.
Still I will offer a few lines on the chips themselves:
It appears to be double side "through" board with surface mounted stuff for the most part. It sounds like a lot and indeed if it was a more modern/complex device it would be horrific to try and reverse engineer but it appears you lucked out here.
The reverse side looks to be nothing more than a few joins.
Most pins seem to be not connected.
The resistors are basic resistors in the ohms range (121 -> 12*10^1= 120.... 010 = 1 ohm by similar method)
The brown banded devices look like ferrite beads and the others are obviously LEDs.
The chip itself appears to come in a few different packages of which one will surely have a readily available socket.
In the end it might be more like $10 and a hour or so from parts on desk to installed in GC but yeah it is possible it seems. You could probably even get away with a bit of stripboard/Veroboard even.
Everything that FAST said seems to be correct as always. The board is pretty simple, and you could just get it away with some non-SMD resistors and ferrite beads, and obviously lots of 30AWG (Kynar) wires.
You should obviously have a way to flash the ATMEL, like having a programmer board.
There's a guy on tehskeen claiming to have been able to flash the ATMEGA8 correctly, but I sincerely have reserves on the way he made it work (powering on and off 10 times? hmmm), but you could still try to PM him and ask some infos.
IMHO, this isn't probably worth your time. If XenoGCs aren't sold anymore, then it could be a good idea, otherwise just grab one, as I seem to remember they were quite cheap.
I bought about 20 chips
and 10 boards
I also tried to reprogram a wiikey
Using AVRDude and home made programmer
Tried different fuse setting and no luck.
Used 2 Gamecubes pre-wired and also tested with XENO GC to make sure everything was wired correctly.
It is either a bad HEX or I don't have the correct fuse setting.