Can anyone recommend an aftermarket, front-derailleur-compatible chainguard (along the lines of the SKS chainboard) that's not made of plastic? Maybe something like the one on the Marin Bridgeway triple?
I doubt it exist over the shelf. It's already hard to find one that's made of plastic. If you really are set on going metal I'd suggest keeping a single chain ring and focus your gearing in the rear (hub, cassette, or hub+cassette combo). Building a custom chain guard will be costly, a pain, and not guaranteed to be a success. If you really want to have some gearing in front you could look at the SchlumpfDrive but at these prices you could also just get a Rohloff. Both should work fine with the standard metal chain guards.