After having to have colic surgery on a very young foal who was one of those little 'ground vacumns', my then-vet recommended that I give psyllium on a regular basis to any foal who behaved the same(and I had several over time...some never even THINK of nosing along the ground, picking up everything they encounter, others seem to do nothing BUT that!) Yes to white salt block and free choice loose minerals, and full access to fresh cool water at all times, of course, but as you said, that doesn't 'appease' some.
It is a chore and isn't 'fun' to do; it's messy and time-consuming, but I did it, and never again had an 'accumulation' colic in a foal. I agree, too--No muzzling of such a young foal.
I used a fairly large dose syringe (about 35 cc; you can order those through any of the livestock supply catalogs, or buy one from your vet), and cut the tip back some to enlarge the hole in the tip-the 'mix' is THICK, and can't be 'pushed through' the tip of a 'regular' syringe. The total dose was about 1 T., crushed to a powder, if your psyllium product is in pellet or crumble form(use a high quality product, that is 'all' psyllium, not a lot of 'fillers'.) You can only 'dose' with about 1/4 to 1/3 of that Tablespoon at one 'go', do NOT add water; on vet's rec, I used a sugar-free 'pancake syrup'. Add enough to make a runny 'paste' (I used an old chopstick to reach down into the syringe and stir-holding a finger over the tip, of course; you have to pour the ingredients in from the 'plunger' end). When mixed, CAREFULLY insert the plunger while GRADUALLY removing your finger from over the tip to release the air pressure as you GRADUALLY turn the whole thing 'upright'--then give to baby a BIT at a time; place far back in mouth and only a bit at a time, allowing them to mouth and swallow each bit. Do this SLOWLY, so as not to cause the foal to choke.
As I said, it's MESSY, and SLOW/time-consuming, for safety. I felt it was worth it--and I never had another foal have an accumulation colic--so REALLY 'worth it', IMO. Ask your vet about it--I found it useful.
Margo