Susan, I have no doubt your customers will be very supportive of more stamp camps. I know they are a lot of work, so maybe you can encourage some of your customers to make a project for a given month's stamp camp. They get to be the star for a day, and you have a little less work getting ready.
OWH is a great organization. I've sent some cards to the troop through our church's "Adopt a Chaplain" program and agree that it feels good to know your cards are going to such worthy recipients and will allow them to share with their loved ones.
I really like your idea about the sentiments. If you aren't planning to sell your sets, this will make it so much easier to find the sentiment you are looking for. But, just in case, maybe stamp each sentiment and note which set it came from, in case you decide to sell and need complete sets.