why not hit it with the rust converter and over coat it with a good old undercoating for a car..............although not sure if it'll leach nasty stuff into the water.Maybe the roll on bedliner stuff would be a bit better???As I'm sure you know there really isn't a off the shelf culvert coating,so whatever you use I'd be sure to check if any nasty chems pose a threat.
As far as drying it goes,I had good results using a leaf blower to dry parts of the bilge on a clam boat before welding and painting...........Again though this is sort of fireball territory,if not worse as there is a guarantee of co emissions in a 40ft tube to be strong enough to KILL you .Maybe compressed air from a compressor (not gas) would be a better choice
Can I ask what purpose the culvert serves?because to be completely honest if it was a non critical area and I inspected it,I too would recommend treating the rust to a customer(boat/ship experience not pond though),yet if it was mine and non critical I'd just keep an eye on it for replacement and budget accordingly.Although that is just me.
