I have not done it but so long as healthy growth is coming out, you are in business. If the branches are too large diameter to graft, you should be able to cut/tip them back and have new smaller growth emerge and then go a standard cleft/wedge. I've also seen jack done with approach graft and Felipe posted a pic of a veneer graft of lakoocha onto jack so you should be able to do a veneer of jack onto jack? Good luck and I believe there is less latex flowing in the cool of the morning.