You probably wouldn't be liable Rob unless the plumber and electrician are working for you. The electrician might be doing what the customer wants, but should still be following his codes for the conditions his work will be effected by.
Normally here in Ontario I require the rough drawings for the plumbing in order to obtain a permit from the city if I'm doing renovations that require me to alter the existing plumbing and walls ... the electical details aren't required for the permit because his licence allows him independant freedom .... which also means the buck stops in his lap.
But ... I agree with you that someone needs to step up to the plate and make sure this project is done right. But as far as liability .... seems like the customer would only be able to sue the contractor ... who is himself.
Oh ... and don't forget to take your camera if you take on this project.
