I think that a borg could beat nethack. I believe some people may already be working on it. In nethack, most advanced characters die to careless moves and boredom. For example, a borg player would never fall downstairs while wielding a cockatrice or choke on his food. A borg player will never put a wand of cancellation in his bag of holding. A borg player always knows his prayer timeout.
I think that a C++ only API is fine. Anyone capable of writing a decent AI would be able to learn C++ if they didn't already. (Or make the API be in the same language the game is written in - whatever that is.)
The problem with a scripted langauge is that they tend to be really inefficient (slow runtime), or they don't offer enough flexibility. For example, Dungeon Siege has a pretty powerful scripting language, but it still isn't enough.
Also notice that offering an API doesn't require compromising CD keys. You can require everyone's client to link to a library that performs a CD key check.