Gold is one of the most inert metals and it does not react with hydrogen gas. That said, I am not sure what the permeability of hydrogen through gold is. I suspect it would be pretty low.
The polymer used for the balloon will likely determine the rate of hydrogen loss. Latex balloons do not retain hydrogen very well. Mylar balloons are a lot less permeable...
Why do you think the polymer would determine the rate of hydrogen loss? I would think it would be a lowest permeability coating which would determine that.
Just like the it is the aluminum coating that gives mylar it's much reduced permeability to gases, I would think that if latex rubber could be successfully coated with gold (or another metal), it too would have low permeability to hydrogen, helium and other gases.