MOT testing guidelines says it's acceptable for the customer to act as an assistant, but, in paragraph 43, page 22393 section 887 appendix 007 revisited, the footnote in pale type clearly states that a parachute must be provided. ( How many solicitors read this board btw ?? )
Seriously though, I personally would only do this if the customer was already experienced ( trade ) but even then I don't like anyone in the workshop for safety reasons. Once had an elderly chap stick his beak in while we were doing a tyre, he walked backwards & fell down the rolling road, nearly broke a leg co's he stepped between a roller and the trip bar which acted as a pivot. B****y lucky we were.
As for the signage, we have them on the outside walls, and also a couple of 'this is a dangerous environment please keep out' signs on the inside.
It's also well worth asking HSE to come over and tell you what you need. This is a logged visit and once a year does you no harm should you ever end up in court. They want to help, not close you down, so bite the bullet and ring.