NXT forging is deterministic, we are not searching for a needle in a haystack. Even with 1 billion forging nodes, you will know with 99.99% (or something close to that) accuracy the next X people that will forge. In the future, base target (difficulty) will simply be a legacy from Bitcoin - which c-f-b mentioned in OP - because why do you need to adjust difficulty when the forging order is already determined? If the order is determined, you simply say "X, broadcast your block at 60s, otherwise you will be penalized." 60 seconds later, "Y, broadcast your block at 60s, otherwise you will be penalized." And so on. In this scenario, X and Y are groups of forgers.

Anyways, I won't claim to understand how any of this works, but the important concept to understand is that forging is deterministic, thus you can have blocks come out in exactly 60 second intervals.