Two Words:
"Hidden conditions" there was no possible way to forsee the conditions inside the walls. If you do not include this clause in your contracts you should start. If not break out the sawzall it is not your fault they closed in the walls prematurely. If that is not an option I agree with triples' method and pick yourself up some 50' snakes and cut them to appropriate lengths, fiberglass snakes are nice but nothing beats a steel snake. Since they are putting up baseboard make all your holes behind that. Hire a temp laborer for all your cuts and cleanup you can't beat $20 per hr and you can tend to more important issues.
Good Luck
Jeff Rose
