A creative Eco-friendly Underlayment for a metal roof?
Red Rosin Paper (which is a bit old-school, but can still be found) would fill the bill, I believe. It's plant based. Seems to be getting sold for floors now, but was the thing people used before housewrap on exterior walls under siding.
However, depending on type and installation of insulation, all you really need is an airspace between the insulation and the roof. You need a vapor barrier between the ceiling and the insulation.
Some sort of cloth could also work, just to contain the top of the insulation and maintain an airspace. Boards also work, but may cost more than you want to spend.
You will want metal screening on the vent openings to discourage rodents (and perhaps other things) from nesting in the insulation.