It's helpful to put all that stuff right there in the boarding agreement, the payment for excess food.
It's harder to know what to do about the problem of the BCS. On one hand, this should 100% be the owner's problem. On the other, it is *you* that are going to have to deal with the animal control cops when someone calls to report a bunch of wasted horses in a field. On the third hand, running a barn is something a person does for love, not for money, and how can anyone stand around watching a bunch of horses on their property stay wasted? Yet, who is going to establish what the minimum-acceptable BCS is, and what happens if a horse gets sick or old, and the BCS drops just from that? I can't see a responsible BO kicking a horse off the property just because it's old. (Yes, I can imagine someone doing that, but I wouldn't consider that person to be "responsible".)
Seems to me the big challenge for the OP is this last bunch of concerns - the question of how to finance the horse's weight gain is the easiest one to answer: pass through those charges as has been suggested. How to write a contract that will not put the BO into an extremely awkward position with respect to watching a horse waste because of an owner that can't or won't pay to feed it properly, and keep animal control off the property, but that won't result in the owner of a sick or old horse getting caught in the same net with the owners of neglected horses.