There's no "special test" for a + rating. So, there shouldn't be any worries there. But, OTOH, one jerkoff thought he was supposed to test it at 5/3 of the 10% overfill pressure (actually, he tested at 4166 psi, when it should have been 3750 psi). Haven't been back to him since, and the cylinder didn't pass the plus rating at the next test. The world is chock full of incompetents.
You can fill in all of the marking if you want to. It's your cylinder. Just fill in and camouflage the "+". You'll be fine. The hydrocheck police won't get you. The only thing that matters to the filler is what he does see, not what he doesn't. He's not going to wonder "is there a '+' under the thick paint of the first letter of his last name right after the hydro stamp?". After all, there's not supposed to be one, and even if there could be, he wouldn't care in the least if it didn't get a "+" stamp. Just don't change the marking to be something different. Then you could be liable. The "+" is invalid anyway, and covering over the marking, giving you a lower fill pressure, should not get you in any hot water, even amongst the strictest hydrocheck police.