Two things. One, I think it actually would violate Reinheitsgebot, which has resisted all kinds of innovations and forced brewers to do cockamamie things to, for example, adjust the pH in their mashes. (No adding lactic acid!--you gotta make it naturally.) I'm not sure what loophole the chemists think there is (that yeast wasn't originally included as an ingredient, maybe?), but I don't see it passing muster.
[At Weihenstephan], undergraduate student researchers have slipped biosynthetic genes for caffeine into beer-brewing yeast so that one day we may drink a pint of beer and defy sleep at the same time. The team has also inserted into the yeast the genes required to make the citrus flavoring limonene. And they’ve tinkered with genes to make xanthohumol, an anticancer agent that additionally blocks hot flashes in menopausal women...
You often see copies of Reinheitsgebot at breweries like this
one at Paulaner in Munich.
But the project is also radical because genetically engineered food is controversial in Germany. Curiously enough, though, the new genetically engineered yeast could provide a loophole for German brewers who might want to experiment with beer additives without disobeying the Reinheitsgebot... [I]n principle, brewers of the future could slip all sorts of interesting flavors into beer through yeast. They’d still be able to add the Reinheitsgebot seal to their bottles because the flavors wouldn’t be inserted as additives.
Secondly, and more importantly, Reinheitsgebot is, in this European Unionized age, more an artifact of culture and a brewing mission statement than an actual law. Brewers adhere to it far more because it puts them in a centuries-long tradition of German brewing and because German drinkers themselves put serious stock into the notion of "purity." There's just no way GMO yeast passes the smell test for being Reinheitsgebot-compliant. I don't think even think Americans would go for it, much less Germans.
But I'll throw it out there. What do you think--designer yeasts that make your beer more citrusy, blocks hot flashes, and gives you a caffeine boost. Kosher? An offense against God?