Hi Rusty,
we used PA38 for 15 years in a flying school. generally we were not over gross, but sometimes a little for the first lesson ( and we should not have been even a little but look in the real world it happens..but you should'nt do it...if there is an accident on the first take off, the first thing the accident investigator is going to do is determine the weight and establish the centre of gravity ).
i even had to refuse a trial lesson one day because the guy was just too heavy. apart from the difficulty of physically getting him into the aircraft, i worked out i could carry no fuel at all with both of in the aircraft so it just was'nt on. he was pi$$ed...but all you can do in that situation is move him up to a bigger aircraft.
regarding your point that gravity wo'nt help....of course it will. surely you know the fuel drains are on the bottom ot the tank. open them and gravity does the rest.
as regards re-using that fuel...we never...ever..ever..did that. the engineering boys got that to wash things down. you can't dump it and you certainly should never put it back in the tank even if it has been drained into something spotless. fuel in JAAland and presumably where you are must only be supplied to aircraft from an approved fuel installation...which your jerrycan is not.!!
for flight testing we had to use PA28 or else in exceptional circumstances ( if weight permitted )we might agree as examiners to split the flight test into two.generally the candidate is told to ensure that as the flight test lasts about two hours to ensure that three hours fuel was on board ( though the latter part of the test is generally conducted at an airfield )...but is possible sometimes to do the navex first, refuel and then do the upper airwork. its far from ideal ( say in a situation where the navex if failed it can create complications )...but to facilitate sometimes one has to try to accomodate otherwise nothing gets done..
the fact you asked shows you are thinking...good for you..
fly safe..
the dean.