A number of people have posed the question - what if there was an aircraft on final?
The answer is that a) the approach controller would have known about the aircraft and given traffic information but more importantly, when on base leg it is imperitive that the pilot looks along the final approach to ensure that there are no aircraft making a longer final approach than them. If there is traffic then you can position the aircraft behind it if required.
Orbits are not legal without ATC clearance...follow the pattern etc. Thus one can never orbit in the circuit at an airfield without ATC i.e. AFISO, A/G etc. What one can do however is leave the circuit and then re-join it in the appropriate manner if required.
There is also no overtaking or undertaking in the circuit thus a citation can easily fly a larger circuit between two PA28s and maintain it's position by not joining final ahead of the first and by the second not turning inside the citation. It's all in the rules of the air.
The 500ft rule does not apply to aircraft taking off or landing. ATC use 400ft when vehicles are on the runway. However if there was a vehicle on the runway and you commenced a missed approach at say 50ft then one could consider the matter of recless operation and endangering property possibly?
That's $.04 !!
DFC