Recently used a "modified" small diameter low nap paint roller to get some epoxy primer inside a rocker panel.. Main intent was to seal up the lower pinch weld seam to prevent water from collecting there. We mixed up epoxy, dumped inside the rear quarter area to let it run inside the rocker, and used the long paint roller to push it all the way to the front.
http://www.garagejournal.com/forum/showpost.php?p=6133310&postcount=2144I would think you could use a threaded rod to hold a paint roller and coat the inside of your tube with epoxy primer in a similar fashion. But as you say, it will be messy..
