I can't see why it would not work. It should be easy enough to cut the line somewhere and put a pump in temporarily. If you end up making it permanent, switch it with a button so it can't be left on.
Another way to try it would be next time you are at the dock
, shove a hose from the dock
up the inlet from the outside and turn the tap on. That would simulate a pump. If you do this, be aware of flooding anything else that is on that line.
On another note, I can't see why the length of inlet line should make pumping hard. Our toilet takes its flush water from the engine
seacock and has around 20 ft of hose. Pumps easily except when the engine
is running , and then needs extra effort.(I have a ballcock in this line and turn it off if we are motoring for a long period incase the head
pump allows an airleak back into the system, but this should not be relevant for you.)
Maybe you should double check your line and seacock just to make sure there is not some other factor (Kink, blockage, airleak) that is causing the problem.