I have been wondering if there is any reason I could not just add a tee to the discharge line and run a hose to a pump out fitting that way
That will work
just fine...in fact, that's how it's done on most tanks
that are plumbed for both pump and overboard
However, I'm thinking that a y-valve MIGHT be better than just a tee...because if you keep the thru-hull open to allow "pass through" flushing
at sea, leaving it open during pumpout is likely to pull in sea water
... Which MIGHT not be a bad idea because it will rinse out the tank. Let's see what others who have experience with it think.