Hi Tim,
I've grafted seedling trees when they were pushing a lot of new growth. I honestly don't recall the time of year, but to the best of my recollection, it was around spring. I used a cleft graft on 3 seedling trees and they all took. One took a year to fruit, the 2nd took 2 years after grafting, and the 3rd has not yet to flower. It may work out next year on it's 3rd year?
I have other seedlings growing for grafting, but I may likely just get rid of. From the fruit I get from my trees and family member trees, there's just too much fruit to consume. Let me know if you need a seedling tree to graft on.
eNorm