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.
Andrew, MASI Treasurer
This message is always under construction: 75-gallon tank; 2, Eheim 2026 filters - one twice broken; Tek Light with 4, 54W T5s (6000K) ; Sand on top of 4:1 sand:clay mixture; Milwaukee CO2 controller; PlantGuild vortex CO2 reactor; pH = 6.6, kH=70mg/l, GH=120mg/l; EI; Flourish excel on 50% weekly water change: AGA Member.