I've now gone back and re-read the Turak scene. There is no description of the scabbard, at all. However, RJ described nearly everything else in this scence as he always does. Turaks fingernails, his clothing, the servants clothing, the silk-screen, etc. The scabbard is noted more than once by RJ, but only in that it's present, with no detailed description.

Fitting my theory, I have a plausible explanation not requiring much twisting. Given the above, it could be argued that the lack of any descriptor for the scabbard would indicate that, in fact, the scabbard has no signifying features, and is rather plain and unadorned.

Move forward to tGS. It's pointed out that Rand recognized the weapon, and not the scabbard itself. Further, it's noted that the scabbard/weapon were gifted to Rand. The biggest stretch is that the comment on "centuries old" is in fact regarding the weapon itself, and not the actual scabbard. This is the biggest leap, as the wording can go either way. But it's not that great a leap, as we know it's the weapon Rand recognizes, and so we can assume it's the weapon that is centuries old.

Now given the plain scabbard from Turak's scene, and given that this was gifted to Rand, I don't find it hard to believe that the Dragon was painted on only recently, after it was decided to present this gift to the Dragon Reborn.