Use a pin nailer. You'll set it on a lower air pressure because the bark is softer. Using test scraps and get the nail heads to sink into the bark but not all the way through. If you pick your nail placements right you shouldn't even have to do any filler touch up; the nail heads will just disappear into the rough bark.
Finish the bark with a flat polyurethane, and be careful not to get the finish on the rest of the wood. Use a 1" brush and push the finish into all the crevices in the bark. The flat finish will keep the bark looking closest to natural. You'll probably want to go with a semi-gloss or satin on the rest of the mantle. Again, experiment on test pieces.