Well i'm sure the RIAA can shut you down if it comes from your machine, you are responsible for what goes on, on your machine. So, if it comes from your machine, saying it wasn't me wont matter to them. to the RIAA and ISP's it's guilty until proven otherwise not the other way around. and if you chose to use passthrough, then your responsible for what passes through, same as the ISP's have to answer for your actions if challanged by the RIAA/MPAA by sending you a letter warning you to stop allowing copywritten material from coming out of your machine.
Who in their right mind would let others upload through their machine, unless they are out of reach of the long arm of the riaa/mpaa, then it might be a good idea.