Unfortunately I have not yet found the text of the official DOC/DOCG decrees on the web. This
http://snipurl.com/1u5yl seems to offer a good overlook however. If you scroll down to Alto Adige/Valle Isarco Veltiner, the info, however says: "Minimum 85% Veltliner, maximum 15% other varieties" without specifying what "Veltliner" exactly is. So Grüner Veltliner would certainly be permitted — but I have not yet encountered or heard about one.
But then I am in no way a specialist of Alto Adige wines. They have a strong following in Western Austria — of course because of historical reasons, since "wine" in Tyrol and Vorarlberg has always meant Südtirol wine —, while they have always been a quantité more than négligeable in the East of the country where wine was grown locally.