Salt sure does get them slobbering! I feed it.
I would abandon the feeder and put it in a small Fortiflex pan. I have one that's about 2' diameter with flared sides about 12" high. I just sprinkle a layer of salt on the bottom of it daily. It's not enough to dump on the ground. Mine are loose in the barn and nose it all over the place, but ok, so far. I have a small feed bin for the minerals that I attached 6" off the floor. I was thinking of getting another one for the salt to put beside it, but found the price discouraging.
I think that a horse likes to get a mouthful with several licks and bring his head up and work on it. That's when the mess starts. I find that with my lowered feeder, the mess does not go up as high as the bars would be. It's in a nice little alcove right by the door, so not a place where they "go", so no gifts left behind for me yet. Maybe you could try a front corner of the stall? You can sprinkle it over their hay as well, if its situated differently vs. the bars and the grain feeder. Before the fortiflex pan, I just offered the salt to them from my supply from a margarine tub. Offered it until they didn't want anymore, then made them go take a drink. A few minutes and we're done. Sometimes, they didn't want any, so I guess that's still free choice, lol.
I like to do this on a daily basis because they seem to go for it, then go for the water. For the same reason that it can also be corrosive to the bars, I also worry about the teeth. I encourage them to drink and right away. Also, I wouldn't throw the salt on the ground, not very environmentally friendly. When I get back into meals, the minerals will be balanced and I will be feeding that individually and probably the salt will go where the store bought minerals have been all summer.....maybe...
And that's another thing that's been bugging me, about feeding minerals instead of free choice. I may try the new minerals free choice. What I've noticed, is that they lick and slide the minerals back in their mouths to swallow, they don't chew....it is stones after all, although sand-like. I'm hesitant to put it in their dinners and have them chewing on it with the rest of it. I'd rather they have the option of eating it the way they'd prefer to.....again with the teeth.
Hope this helps with some ideas.