Fast fibre has been my saviour for an extremely fussy horse. I mix it in a separate container with supplements/danilon/whatever, then mix really thoroughly into a handful of feed, again thoroughly mixed with a handful of readigrass. It mostly works.
I had one horse who was truly impossible to get bute (or any medication) into him. The only thing that worked was to mix the bute with a teaspoonful of toothpaste, put it on a wooden spoon and wipe it onto his tongue as far back into his mouth as I could reach.
A friend of mine who works at the vets gave me this tip; mix it with baby food then syringe. Worked for my youngster. I think I topped and tailed the mixed baby food with some undoctored baby food in the syringe!