Hello folks, I can't really believe it worked, but I managed to replace the board textures with the earlier ones featured in the "Winter 2001 Jampack" Demo version. Take a look at a few examples:
A lot of boards use a blank texture with the word "board" on it as a placeholder. Unfortunately I am unable to unpack the .ssh texture files and replace those with the final boards. There is a Program called "eagt", but it keeps crashing when I try to open anything but the splashscreen texture
Brodi has almost no textured boards (I think there's 2) Elise, Mac, JP & Kaori mostly (or completely) use their SSX Original boards. And the UBERBOARD looks like the last board before it for each character.
Edit: Also, Seeiah's board shown in the introduction video is a freestyle one, not an Alpine. (Maybe the texture is linked to the wrong board ? Or Seeiah was an Alpine Character in the beta stages... who knows)