What is the fire resistance on a corrugated iron wall?
My intuition tells me the exterior would not provide "1-hour" fire protection. Although the metal cladding certainly would not burn, it will readily conduct heat.
The gypsum/plaster-board is a very poor heat transfer medium and that along with the fire-resistant paper on it is what gives it it's fire rating.

You should start to think about maybe stucco or plaster over the cladding (yes, it can be done) which if properly applied would be great for fire protection (although rating it would be a guess). Or removing the cladding and adding a layer of the plaster board behind it.