Good to know a developer responded, and the "hard coded" aspect makes me think the codec is built in as part of the XBox SDK developers use (just a guess). The update required would be one involving recompiling Shadowrun for the 360 against a newer SDK then pushing out the update. Understandably difficult when the developers and studio are long gone.
I do have to wonder though why Games for Windows Live can't step back to using the older codec when it recognizes Shadowrun (or the other titles) are being played. Sadly it's probably a numbers game, where Wedge The Jedi can't justify the time needed to implement the fallback, have it tested, and deployed on the PC side. Just not enough users to really make it worth while. Which sadly turns into a vicious cycle where there are less users due to missing the ability to communicate.