BIG no-no! Don't even think of doing it.
Only toilets designed by the mfr to use pressurized flush water can safely be connected to the onboard fresh water system. NO raw water toilet should ever be connected to the boat's fresh water system...it cannot be done without risk of polluting the potable water supply and damage to the toilet. Teeing into a water line in the head would mean that the water in the line is pressurized--there won't be any water in it unless the pump is on TO pressurize it. Rubber parts in manual toilets are designed to PULL water in, they're not made to withstand pressurized water being pushed through the pump...so teeing into a pressurized line will turn the toilet into a fountain.Teeing into the fresh water system ahead of the pump is just asking for e-coli in your potable water...there is no check valve that can prevent bacteria from the bowl from migrating into the water tank.If you want to stick with a manual toilet, there are only two safe ways to flush with fresh water: 1) use the shower head or a cup, or 2) install another water tank dedicated solely to supplying water to the toilet. Or, replace the toilet with one designed by the mfr to use pressurized flush water.However, if teeing your head intake into the head sink drain isn't practical, there is a much easier way to flush the sea water out of your existing system than removing the intake hose from the thru-hull: using a 3/4" y-valve (which you can probly get from any hardware store) ...tee a piece of hose long enough to stick into a gallon milk jug into your head intake line in a location that is accessible. Close the seacock, stick the hose into the milk jug, turn the y-valve, flush the toilet. Even if the only place you can tee into is in the bilge, moving a y-valve handle sure beats taking a hose on and off a thru-hull.