Nah, I'd take another, and more humorous route! They embark, and simply arrive on the other side sans iron/steel.
Ride through, come out, no iron of any kind. Knights in doublets, with threws falling down (if they say they had an iron belt buckle), no weapons, perhaps falling off their horses if there was iron in the buckles for the saddle, or horses roaming if their tack was iron. Hilarity ensues!
THEN, you bring out the mischievous brownies and pixies.
SDLeary