If any a/c wants to fly in RVSM airspace then it needs approval which involves carrying the correct fully functional equipment. There are some airlines with non-approved aircraft, ie City Ireland (dont know the proper airline name) B146's but for the sake of a couple of 1000 feet I don't think they need to bother. There are older aircraft compliant, I think even John Travolta's B707 is approved! As Widger says some a/c can get exemption, mainly military flights, but these will only be accepted subject to workload. Formation flights CANNOT be RVSM approved under any circumstance.