Hi Quincy,
I think all you're trying to do is make a valve that is integral with a water reservoir. Your idea will work in theory, but it requires proper design of the valve seating surface and guidance of the bar. Like any valve, there are many aspects you need to look at such as seating loads, material compatibility, stresses, actuation forces, etc... I would consider your idea and any idea like it to be the starting point and go from there. So the first thing you should do in designing ANY mechanical device is to consider various ways it can be done and try to understand what the benefits and drawbacks are that are inherent to the various designs. In this case, the benefit might be simplicity and the potential to make the seating surface pressure energized. Drawbacks might include the fact the bar will be drawn across the seating surface which has the potential of scoring the surface and allowing leakage. I'll toss out a few other ideas...
You might have a design with the hole being horizontal instead of vertical and have the valve stem (bar) oriented directly above the hole so pushing down covers the hole and lifting up exposes it (or visa versa if you wanted). This design can be pressure energized or pressure balanced. The bar would of course be guided. The bar could have an actuator (automatic or manual) that pushed directly down on it or the actuator could be a screw type mechanism such as most common globe valves have.
Another option would be to make the bar cylindrical with a hole in it like a ball valve and guide the end so that rotating the cylindrical bar covers or uncovers the hole. Like ball valves, and the valve you suggested, the seat can be pressure energized but consideration must be given to scoring the seat.
