The underbody is painted and galvanized, so that isn't a big concern.
The pieces most susceptible to rust on the Gen2 are the brackets that hold various pieces underneath. I've seen brake lines rust FROM THE INSIDE from moisture, if brake fluid isn't flushed periodically. Honda recommends every 3 years regardless of mileage.
Good Practice is to wash the underbody weekly in salty areas...and a yearly inspection check on a lift is good, as well. (That way if the lines appear to be rusting, they can be coated with paint or undercoating)
Copper isn't a good material for hydraulic lines.