I've been wondering the same thing. It sounds like they're still years away from having it on the market, but when it finally does, the GB will HAVE to rule on the issue. My guess is that it will become a "conscience matter". Honestly, from the GB's and WT Legal's perspective, manufactured blood is the most ideal outcome for them.
The current blood policy is such a tangle of contridictions, no one can clearly explain or defend it as a whole. The WT will get sued left and right if they ever officially abandon it as a doctrine, so allowing manufactured blood would solve all their problems-- they could continue with the ban on "real" blood, but in reality JWs could accept this manufactured blood by the gallon, if needed.