I have used an equiboot on his bad foot, but found that the mud was still able to get in to the hole in his foot. So I tried using a pad of cotton wool in the hole (which was sprayed with an anti-bacterial lotion which I got from my vet) then I bound the foot with sticking plaster tape, and then put the boot on. When I brought him in from the field I hosed the foot out with clean running water, to ensure that all the mud was out, and in the early stages soaked the foot in warm salt water. I dried the foot and applyed the anti-bacterial spray. The foot was then left undressed all night so that the air could get to it. I had to buy an extra large boot because he had an egg bar shoe fitted so the boot wasn't used instead of a shoe but as well as.
Tim was fed on a feed called lo-cal (which because he wasn't doing any work then he didn't need and energy feed). Lo-cal is made by a feed company called Baileys. It has been developed to provide a horse with all the vitamins that they require on a daily basis with out having to feed huge amounts (Tim is 17.0hh and has approximately 1 1/2 pounds per day).
Yes I do think it is genetic and passed down (Tims sire Louella Weidekreis was put down due to laminitis) and as I mentioned Tim had these cracks in both front feet from being a very young foal.
I had a similar story 2 years ago with the other foot, but that didn't drag on as long.
The good news is though, both front feet (touch wood) are now completely free from cracks.
You might want to try this site it gives some useful information and sand cracks. This is where I think I got the information from for a multi-vitamin supplement www.equilife.co.uk
or try www.hoofcare.com
Hope this is of some use,