I can't help you with a paste mineral/vitamin mix but I was wondering why you felt a paste was necessary? I am raising an orphan and according to my vet the milk replacer is very well balanced for them and when they get older and start to eat solid food more, any good foal ration and pasture (or good leafy hay) will supply them with all they need. What I have been told was that many people over supplement orphans in an attempt to make up for the lack of a dam, if the foal is developing well, is healthy and energetic, there is no reason to feed them anything you wouldn't feed any foal as it grows except you supply the milk.