My FMS doesn't know about the mountains, but my EPGWS does. Actually, so do the Mexican controllers. The controllers just assume you know about them and are perfectly capable of avoiding them. Besides, you didn't have any mountains in your theoretical descent problem.
My point is, you act as if low-time pilots can't figure out crossing restrictions; I disagree. Rules of thumb for descent don't care what kind of plane you're flying, just how good your math skills are. Anybody who can divide six by three can also divide 33 by three. Not that there's any reason to do the math on that, given that the FMS is capable of figuring out descent rate to a degree so precise you won't even be able to fly it.