There are indeed very few kosher wineries in Alsace unfortunately, actually most kosher french wines are made by non-kosher wineries in separate tanks over-watched by Kashrut organizations such as the Beth Din of Paris and the OU so visiting the facilities and tasting the kosher wines is rarely possible for the kosher drinker. One of the very few exceptions might be Vignobles David but that's in Cotes du Rhone which is quite far from Strasbourg/Metz. That said you may want to call Michel Grumbach who owns and runs a kosher catering in Strasbourg called Aux Mille Saveurs, perhaps he'll have more information: (+33) 88 32 44 62 / (+33) 06 80 00 52 81.
Wishing you a pleasant stay.
Consultant with Netofa Winery - Founder of KWSE -http://facebook.com/groups/kosherwinesharingandexperiences/ - Wine writer with ariskosherwine.eu/ and Israpresse.net