Ouch. Sounds like your in a tough spot. As Im sure you know, the only way to do this correctly is to gain access to the leaking area, and fix it correctly by replacing the leaking area. Or gaining access to the upper and lower portion and rerouting the pipe. Somebody made a big boo boo. Fist off, the pipe should not have been poured directly in the concrete (or was it, you say it is blocks, so they may be able to break out some blocks, and replace the blocks without any major sturctural damage?), it should have been sleeved. Second, this leak should have been found during testing of the plumbing drainage/venting system. The fact that the builder went bust, makes this extremely hard on you. The fact that the plumber says it will seal itself, only says to me that your plumber should not be allowed to do plumbing. I hate to say it, and almost never will, but I think you may have a ligitimate lawsuit on your hands. Somebody needs to make this right, and it shouldn't cost you a dime. I certainly hope you had a licensed contractor do the work, and I hope he hired licensed subs, otherwise this could be a real nitemare for you. Good luck, and please let us know how things turn out.
I would like to know what the leaking pipe feeds. You may really have a chance of rerouting the pipe. It may mean building a small chase, but much better than tearing into that concrete support beam.
One last thought. These things do happen. Give the plumber a chance to make it right. But, if he just isn't willing to make things right, and correctly, or you find that the cuase of the leak is not the plumbers fualt, but is the long gone builders fualt, then you need to decide if you want to get someone else in there to get the job done, at your cost, or if you want to go through a lengthy court decision. This really is a tough one. Wishing you the best.