I've been experimenting with Gold/Silver/Variegated leafs in my finishes recently. Bought the leaf and glue from Michaels.
Below is a pic at my first experiment - the "Chevrolet" lettering has the silver leaf and a thick coat of Testor's One Coat Lacquer.
I don't see any way around applying the leaf over the decal unless it is on the model's intended finish.
Since you want to apply leaf over the decal, you probably want to apply the decal to the model and let cure. Then a coat of clear, let cure. THEN apply your leaf, add more clear.
Its going to be time consuming and require patience, but the results will be worth it, I'm sure.