I think placing it in an area of gentle water current, then adjusting the size of the opening will work great. Obviously the opening in the syringe is a little on the small side, of course much depends on the size of the tank that is to be dosed.
Last night I took it up a notch. Used two 12ml measuring tubes (they come with Red Sea Nitrate/Phosphate tests), attached them to a rubber sucker, filled them with one tsp KNO3 and K2SO4 respectively, and put them into the tank. This morning I noticed a slight reduction... not sure if it dissolves or compacted some over night. I think this might just work great...
BTW one tsp of powder translates into about 4ml of volume in the test tube. So the 12ml tubes would hold ~3 tsp, enough for 2 weeks of N or 3 weeks of K for my 100gal tank.
The neat thing is that this can be fairly small, and unobtrusive... made all out of glass, you won't notice much. Just put it in a place where you can observe the level of salt.