It would be hard to imagine a scenario where you could convince a court that a renter could not have reasonably known that the C of A had expired because it would beg the question what reasonable steps had been taken to satisfy himself that the aircraft was fit for flight? If the paperwork is not available/the pilot failed to look at it how could it be said that all reasonable steps were taken and he/she was properly satisfied?
I agree, but some court rulings (in the UK) have been rather perverse.
Taking the ANO literally, a PIC is strictly responsible for a long list of stuff, but people have still got off where the school (or the operator, if self fly hire) was poor at record keeping.