ass gets used with naked all the time. It's breasts, topless, and bottomless, nopan, and no_bra that aren't supposed to be, because they indicate different levels of exposure while still being partially dressed, and many naked_apron pics depict the front of the girl rather than her back, so I'd say that it doesn't make sense to assume that naked_apron implies ass.

Obviously, there are a number of different ways to that we could go about this, but in general, I would hope that we wouldn't be trying to make this overly complicated. And treating naked_apron, naked_cape, and naked_ribbon the same as naked as far as which tags they disallow is just simpler. That's certainly what I've been doing. The only difference is that I'd apply tags like pantsu and bra to a naked_apron pic if the girl was wearing them (though if she's wearing more clothes than that, it's really just apron and not naked_apron, and we don't have a tag for that anymore).