If using threaded inserts is way too involved, I would go with what ACG said but with a variation:

Use epoxy or something like JB Weld to rebuild the posts and with a screw inserted so it molds to the threads. To insure that the epoxy releases the screw, coat it with a "mold release agent"; something like a thin coat of silicone grease so the epoxy won't adhere to it.

You will be visiting the inside of the tank's hull from time to time so something must be done to make the screws more user friendly.