The purpose of the black cylinder is to keep the tube aligned, so it should be safe to gut it in order to access the screws. You may be able to drill holes in order to access the screws, or just remove the black part entirely (using a dremel, knife, some sort of saw, etc.; be careful obviously) and find an alternative option to keep it aligned to the tube. Alternatively, you could try using a plug insert (which is basically a dead-end barb, kind of looks like this
), and then clamping down with a hose clamp, although you'll need to attach something else to keep the tube aligned. If you can't find a plug insert of that size, you can just use a barb to NPT adapter (which looks something like this
and a threaded endcap. (Make sure to use plenty of teflon tape for the thread, of course.)
Some other members (who've actually worked with these bladders) may be able to offer better solutions, but this is the best I can think of right now.