To answer the original question, unfortunately there is nothing in the rules to prevent it. One company that I flew for, possibly the same one as jafo, thought nothing of expecting a crew to position by hire car from the south of England to Manchester and then operate, or do the same journey after a tiring duty day. I was once on standby at Nantes and operations called, suggesting that my crew position by hire car to Nice and then operate. I declined.
It's legal, it's cheap and it's stupid - a microcosm of today's airline ethos.