Correct me if I'm wrong but wouldn't this still be blood? The only difference would be the manner in which it is produced, but it would still be blood and since JWs say no blood can be taken into the system then they would reject it. Consider a parrallel situation: The JW organization considers artificial insemination with sperm donated from someone you're not married to as fornication - even though no sexual intercourse is involved. If that's their stance on artificial insemination I don't see how they would approve of blood transfusions just because the blood is produced, or rather, its production is triggered, by artificial means. Once it's actual blood they will refuse it.
However, the Watchtower has been known to be inconsistent in it's reasoning, applying X reasoning to Y situation but then inconsistently turning around and applying A reasoning to y situation. Consider for examply their position on marrying young as compared to being baptized young. They have no problems with minors making what they consider to be the most important decision in life - baptism - with severe repurcussions if they change their minds later in adulthood; but they say young adults should wait until they're older an more mature before entering into marriage. Two diametrically opposite positions for two comparable situations, both involving making important life decisions at a young age. It is noteworthy that the two inconsistent positions benefit the organization.
So if the organization stands to benefit by allowing the transfusion of stem-cell blood then they will inconsistently permit it and use some kind of twisted logic to deceive the rank and file into thinking there's a huge difference between stem-cell blood and normal blood. If the organization is looking for a way out of bad publicity due to the refusal of transfusions then they might see stem-cell blood as an opportunity to get out of the blood quagmire by permitting JWs to have it. Then slowly they could just start turning a blind eye to the whole blood issue and not have to fear lawsuits from families who died refusing natural blood in the past because there was no stem cell blood in the past to give them. They can definitely see it as their window out of the blood issue.