Ive used Polyvine Decorators dead flat acrylic varnish and Craig & Rose Flat Varnish over hand prints to protect them but never tried it over a metallic. Its possible but could taint the metallic effect, as always, test a small inconspicuous area first and see what the results are.
I am awaiting an answer from Polyvine on the same lines. If you look at their product range, they have metallic glitter and such ready mixed in a glaze. So the metallic may be alright. But as Russ says, test, I have noticed that Polyvine dead flat can darken dark ground colours.