Although this isn't as simple as an open vial, I wonder if you could use an airstone, some tubing and your container of concentrate. Let the head slowly push the concentrate through the airstone or just saturate the airstone. If you have any water movement, that should be sufficient to dose the tank. Plus, small internal filters are inexpensive and could be used as well to help out if your water is somewhat stagnent. If your aquarium is 18" (457 mm) tall and your container is a quarter of that, you won't get that much of difference in head so the dosing shouldn't vary that much.
I think I understand what you are suggesting, but what I like most about what I am trying to do is the simplicity... nothing to install on top of the tank, no airlines, pumps, things that can clog up quickly... just a partially covered vial, filled with salt. Plus, very easy to see if it is still dosing, or about to run out.
I will post some pics soon, maybe that will help to demonstrate the very simple concept.