Suppose a target plan converts to a profit sharing. Now they want to add in-service withdrawals. Are there any regulations specifically allowing (or not allowing) this?

A target benefit plan is a specific type of money purchase pension plan. Therefore Revenue Ruling 94-76 applies: one cannot allow in-service withdrawals from the money that used to be in the target benefit plan (and related investment earnings). I don't believe that the new 411(d)(6) regulations change this conclusion.

You should be able to get to Rev. Rul 94-76 from here:
http://www.taxlinks.com/rulings/findinglis...evrulmaster.htm