The wood liners of storage magazines is part and parcel of the "non sparking" mentality. That's why the box is supposed to be lined with wood. Steel = bad.
IINM, wood isn't used in powder storage box construction because it's non-sparking, but because of it's insulating properties. Not only will steel spark, it has a high thermal conductivity.
In reality, the NFPA standards don't have any requirements, beyond maintaining the original containers, for storing up to 20 lbs of smokeless powder in residences. So, you can keep limited smokeless anywhere you want.
The standards for shipping differ from those for storage. I know of cases where gun shops have kept their powder in an on-site trailer because of less restrictive transportation standards. It doesn't come under the jurisdiction of the local Fire Marshal until it's removed from the transport vehicle to a building.