I'm working on the same thing as you right now! I would love to teach Meg to fetch, but she came to me very uninterested in any toys. She now loves to play with her stuffed animals, but clearly has very little fetching instinct.
It sounds like I started the same way as you, c/t for touching the ball. She really gets the clicker, so I pretty quickly (within one session) started only clicking for her mouth on the ball. She's a big fan of batting at it with her front paws
. I think the treat is the way to go for now. Hopefully someday, the "reward" will be another toss of the ball. For now, since the click marks the moment you are rewarding, you can still reward her for picking up the ball - it shouldn't matter that she has to drop the ball to get the treat. In four or so sessions, Meg will reliably chase the ball a short distance (10-15 feet) and pick it up. Right now, she tends to drop it when I call her back; that's our next step. I'll tell her she forgot her ball, and she'll run back, pick it up, then drop it to come back to me.
This is the first time I've taught fetch (Well, to a dog. I taught my horse to fetch a frisbee.), so someone else may have better, conflicting advice, but I've been keeping the balls picked up when I'm not ready to play. Like you, I was worried her interest would fade if there wasn't any rewarding.
Good luck! Glad to know someone else is in the same boat as me.