Access aside, I'm thinking that one of the major obstacles to coating is proper preparation of the tank, along with it's
current level & type of finish. Since it's commonly necessary to both clean, & abrade steel prior to the application of many
epoxy coatings. Plus some of them (coatings) require post cures as well, though not all. In order to prevent them from contaminating the tank's contents. And a fairly even coating of epoxy would be needed for it to cure well enough to
work properly. Which, without good access in order to apply it could be a problem.
The other thing to think on if considering using a bladder, is that assuming that you can cut a proper access hole to insert it, & plumb it. Is the question of what will happen to the bladder as the
current steel tank corrodes? Since there will most likely be some sharp edges, & abrasive surfaces created as the steel rusts. Which of course could tear up a bladder tank as it wiggles around with the boat's motion.
Edit: Did the epoxy that you applied suggest that you remove the galvanizing? And might you have a link to the stuff that you tried? I'm curious to know more about it.