I mounted a salt water spigot fitting along the edge of my forward deck ...far enough inside the toe rail to enable me to pass a mop...and I plumbed exactly as you describe. No metal plate off center. I also put in a TEE under my forward cabin floor to enable me to easily drain the hose connection going to the salt water washdown deck spigot for easy winterizing. My toilet remains operatinal all winter as I leave on heat.
On my 1972 48 YF there IS an aluminum reinforcement plate, about 1/4"thick at the center of the deck encased in the deck laminate. I found out about that when I drilled to mount a new anchor pulpit and broke my drill. In that pulpit which I made myself, I ran copper tubing inside with a spray head at the forward end to rinse chain automatically via flybridge mounted pump switch. Just before the anchor winch mounting at the aft end, I used an elbow to extend the copper water line down into the rope hatch area (under the deck) from the underside of the pulpit...That goes to a separate pump off the same toilet intake line...99% of the time that's all that is necessary to clean the all chain rode.
The pulpit washdown arrangement is best, but more difficult to install, and if you have a crew emember to schlep up front, good enough. (My Yorkshire terrier is not interested in such deck duty!! especially in rain,sleet,snow.) Also in breezy days the forward looking nozzle sprays back onto the forward deck, so sometimes I have to turn the boat parallel to the oncoming wind...
Most fun: other boaters watching me wash off my chain withnobody up front... mouths agape!!! Have had several inquiries "How did you do that?"
