well, just got a reply back from the support site for xbox live, and i think ill just quote it:
hank you for writing Xbox Customer Support!
We appreciate you taking the time to write us with your concerns and value you as an Xbox customer. We do apologize for any inconveniences that you might have.
I understand that you have purchased a subscription card for Xbox 360 and you own an original Xbox console. When you try to enter the code, you receive an error message: Unable to continue due to technical difficulties. Please try again later.
We would like to apologize for the misunderstanding. The Xbox 360 Gold subscription card can only be used for Xbox 360 console and not for original Xbox console. Please check the following link to look for the availability of Xbox Live subscription for original console. http://www.xbox.com/en-US/hardware/wheretobuy/?WT.svl=nav
cant really find anything usefull on the site provided... u might be able to sign up on a 360 then bring it back to the original or something but i have no idea...