I'm sorry, but I feel that this is just to big of a misstep to not comment. Just because you say this is true does not mean it is so. The United States didn't go through some major change in the 12 years between Sonic SMS and Super Mario Advanced 4 that completely changed the nature of UPC codes. The European GBA UPC example logically proves that the very nature of UPCs is not indicator of region but arbitrarily changed or kept. A counterexample is the proof of the arbitrary nature of UPCs. You can't change that, no matter how hard you will it so.
Additionally, if we as a video gaming community are willing to accept that UPC is indeed indicative of region for the SMS due to the fact that Sega was consistent with it's regional UPC numbering (evidenced by NeoZeedeater), then we must take this through to it's logical conclusion and accept that multiple video game regional set lists would now be incomplete. If we accept that UPC means something for Sonic, suddenly European copies of games for the DS with UPC stickers in America become essential for American lists.
There is no logical reason to do otherwise.
EDIT: Changed phrase "games for the GBA with UPC sticker in America" to "games for the DS with UPC sticker in America," although this would also have weird effect on the GBA game mentioned as well.