Hook up the drain to a clear hose and swing the hose back up into the tank. for the R22 I'd put 10 litres in at a time then line up the broomstick beside the tube and mark away.
The syndicate I was in thought about doing it but we decided it was only a matter of time before somebody damaged the indicating system and created more headaches. I think you will find that dipsticks have been avoided for obvious reasons.....Frank would not like the idea of any foreign objects being put into Robbie tanks. If you are the sole pilot and are pretty careful it may work well for you though.