I don't know enough to say this but I can ask it: If being ultra-orthodox exempts you from service, doesn't that provide an incentive to become ultra-orthodox? So will this cause more people to fall out of ultraorthodoxy? Or is there no joining involved, just being born into it?
Anyone with a Jewish mother is considered a Jew and can choose to life the religion to that extent. However i am quite sure you will need to have made the decision and devoted your time to the necessary studies years before military service for the exemption to be granted.
Think of it this way. Any christian can decide to move to a monastery as a priest or nun. But if he wants to do that not to get drafted he should do that a bit earlier than the draft date.
I expect a lot were raised as such by their parents/community, which isolates them from society as a whole. It will be interesting to see how it all plays out once they get exposure to other mindsets. And gurls.