Sure you could! No, seriously--I would bet you would have no trouble building the interface board even if you've never soldered something in your life, if you start from the premade board (http://www.seeedstudio.com/depot/orbshield-p-280.html) and follow the directions on the site (http://orbduino.sourceforge.net/).I don't have the knowledge to make what you guys are making here but am curious if you are at the stage of taking orders for this adapter and how much it would be.
I don't sell the boards directly (and make no profit off them--I had Seeed Studio do the prototype and they have a few extra). Cost is $6.50 for the board, plus a handful of parts you can probably get from Radio Shack (about $20?), and you need an Arduino microcontroller board (about $30?). So unfortunately you are looking at about $60 to bring the old orb back to life, with arguably more features than ever before, but it's not a trivial amount of money (the good news is it works with the desk-mounted Spaceball devices, which are turning out to be even better gaming devices than the orb, in my opinion).
But seriously, if you are nervous about building one, it's dead easy. I've built a few for people in the past, but unless I buy all five of the remaining boards at Seeed, stuff/build them, and sell them with a labor overhead, it's just a bit too much trouble (although it would force me to fix the minor design flaws and order a new set, I guess).
I'm almost done with the next "revision" of the software (just updating with the fixes Ron and I have hammered out) and when that's done and the Magellan is added I may try and get a bit more publicity and see if more folks are interested--at the moment there are five of these adaptors in existence (heh) so numbers are still very small.