No; that's the short, unfortunate answer. Here's the problem: only two groups of people know the ZEMs. One group is the devs, and (keeping to the classic spirit) they refuse to release them.
The other group are players who are certifiably insane, because you'd have to be to figure out a ZEM without hacking P99 or being a dev. To do so you'd have to have character A solo (at least) one level, while staying entirely in a single zone, and always killing mobs of the exact same level. Then you'd have to have an identical character B do the same thing, with the same exact level of mobs, in another zone. Both would have to keep track of exactly how many of those "exactly the same level" mobs each one killed.
Once you did that you could divide one mob kill count by the other to determine the
relative ZEM of those two zones to each other, but that still wouldn't give you either zone's ZEM. To get that you'd have to repeat the experiment across enough zones that you could be certain a subset of those zones were baseline zones (ie. ZEM 75 / 100% XP). Only once you had that could you finally start determining a zone's ZEM by comparing it's relative ZEM to the baseline zones.
... but of course almost no one has the patience to kill only mobs of a certain level in a single zone for even a single level, let alone to do the same thing for many different pairs of characters (for many levels worth of leveling) across many zones, especially at higher levels. Even just doing one relatively low-level character for one level is super difficult because most mobs have level ranges, and when you're leveling you don't want to fight white cons (ie. the only mobs whose level you can be certain of). To kill blue mobs of the same level consistently you'd need another slightly lower character sitting next to you conning everything.
On the positive side, I did file
this bug, and hypothetically IF (big if) it ever gets resolved we might at least get some clues.
P.S. In case anyone is thinking "what if I used ShowEQ?", even a cheater who could see the exact XP awarded for each mob could only speed things up so much: they'd still have to do the same process, they just wouldn't need a full level worth of killing. But even that increase wouldn't make things all
that much faster, and your progress would slow down a ton the moment you got banned.