I finally bit the bullet and picked up the current version from the Blackbird website. I found that the reg #'s are baked into all but one texture, which I found quite... well... I've puked nicer colors. Since I suck at painting from scratch, I made a copy of the C-GTER texture folder on my desktop. I did a little trickery to mask over the baked-on reg, then renamed that folder to 'NOTAIL' - which is where the original no-registration colors are kept. I deleted the original (I have a backup copy of the entire plane stored), then dropped in the replacement folder. Now I have the nice blue hues of the C-GTER texture, but I get to use my own "N721TC" registration.
My only real gripe about this package is that there's no OAT gauge that I can find. I normally don't fly in icing conditions anyway, but it would be nice to have the gauge so I can check when things are close to cold enough.