I suppose it could be a potential problem with adding more moisture but you have moisture in the air anyway which can rust your ductwork. Hopefully all you are going to do is use a humidifier to maintain a comfort level of 30% to 50% humidity. As long as the air inside the duct never reaches the dewpoint temperature(the temp where the humidity reaches 100%) so water condenses on your ductwork then I don't see a big problem. Insulating your ductwork will help this as well as using galvanizing on the duct work. I'd like to hear from anyone in Florida or those in high humidity areas (who are concerned about dehumidifying) to see if rusting duct work is an issue.
Remember nothing lasts forever. Your ducting could rust from moisture in the basement or attic or from salt air around the coasts(which is why cars rust so quick around the coast).
You ask about potential problems, everyday I get up I potentially could get cancer, and everytime I walk out the front door there is a potential problem I could get run over by a truck or struck by lightning. lol Good luck.