This could be quite interesting and I'm guessing anticlimatic and dissappointing. On the other hand, it could be quite messy too.
If I'm not mistaken, bottle conditioned beer requires a headspace to work properly. However, with a bag that can expand, I don't think it will work. The amount of headspace needed seems that it would be important. Your bags can probably expand quite a bit, due to the carbonation. Since they aren't rigid like a bottle is, they probably won't build up enough pressure to get the C02 to dissolve into the beer. But, then again, I could be wrong.
I'm sure they'll swell up some, like a smack pak, but they may not swell up to the point of carbonation. IHowever, if the bag reaches it's elastic limit, then I suspect they'll burst open. I doubt that they are as strong as a glass bottle.
Let's us know how it turns out. I'm predicting a burst open bag and a mess.
P.S.
You should start a poll and see what people think will happen and then let us know if the poll was accurate.