Substitute pig trotter for powdered gelatine in an aspic
I think the biggest problem is your not going to be able to judge how much gelatin any given pigs foot is going to produce, hence the recipe calling for extracted gelatin to sort of foolproof the recipe. But if your going to try and make it with just the pig's feet I would suggest having a look at this recipe here for "Trotter gear" 

http://www.seriouseats.com/recipes/2011/03/recipe-for-a-healthy-jar-of-trotter-gear.html

Anything coming from Fergus Henderson is going to be awesome. It recommends 6 feet and that recipe produces a pot's worth of stock so I think that will suffice for you.