I`d have to agree with Chap2212 that the best bet seems to be Stéphane Damizet from the 2002/03 season France championship. Although imho I dont see much demand for his signature, or at least as much as if it were the artist themselves. Also The signer themselves defaced the card which would take value away. It reminds me of Gretzky signing cards then bending corners to reduce value. Again imho the card would be worth more unsigned and unaltered being it’s from such an older set and those are harder to come by now (3rd edition/revised) The card itself before that would be worth $20 in NM/M.
You might be able ot try and tack 20-30% on top of that if you are very lucky and someone doesn’t mind the defacement on the card. Being it was only the France championships there would only be a small market as it wasn’t the Prix or the Word championships. Again this is just imho and mean no disrespect.
He wasn’t really a known player and his win losses weren’t all that noticable either. The France Championships seems more of a luck of lack of strong competition as far as I can tell.
http://mtgstats.com/PlayerDetail.aspx?PlayerID=38948
As far as verifying the signature your best bet would be getting in touch with him and verifying he signed that particular card. Even then you’d want to get some letter possibly signed off on by a notary or card grading company to authenticate. Otherwise as a collector on dollar values it’s not worth much more than the stock the card itself is on.