I know that Amy West/Ontario Elite Hunter Sales sells a lot of custom foals. I know that she is still in the hospital from her accident, but she used to have a lot of info. on her website about how her custom foal program worked.
I would guess that there is a huge variation in how different programs structure a plan like that, so you could probably write up a contract however you desired.
Not something I will ever venture into, so can't help more!