Interesting idea, though not without its problems. Being so irregular in shape, Nightfall might not be a very stable anchor mass (remember those gravitational anomalies that made ACC move Sri Lanka to the equator as the fictional Taprobane?). I'd also be concerned about where the tower materials were going to come from, but I guess we could presume that suitably strong stuff has been invented.
Alternatively, how about using Nightfall as the basis for an O'Neill-style space station/colony? Prometheus would be a tiny little tin can by comparison. If we used the original idea, Superman could place the water bags inside the asteroid, and his heat vision could even help the melting process -- in fact, he'd be useful in smoothing out irregularities so that the finished "cylinder" was even in thickness, before and after the water bags blew. And then he'd be needed (or wanted) to cut hatches, help move in the ecology, etc., etc. I suspect that putting it in the L4 or L5 positions might be a bit too advanced for EPRAD, but it'd make one heck of an orbital habitat.
Failing that, simply mine the thing! Place explosives and carve it into managable chunks which could be parked in a convenient orbit. What's the current price of nickel? Bet it would drop with a 17-mile long asteroid made of nickel-iron available! And who knows? It might have other, more valuable minerals inside.
