Go, Read: Brian Cronin's Article On DC's Ownership Of Certain Characters From The ABC Line
I'm grateful for Brian Cronin engaging with the issue of why DC owns certain characters from the America's Best Comics line (Promethea, Tom Strong) but apparently not others (those from the League Of Extraordinary Gentlemen). Basically, LOEG was developed along a different track with a different contractual result, but shared the ABC name for what I suspect was marketing reasons.
I'm not sure why the question didn't occur to me before I was asked a few times and went a-googling. I think maybe it's because I still adhere to the notion that you can be contractually able to do something but not compelled, so I tend to focus on the decision-making that drives a deal rather than the legal parameters that shape a deal. I'll do better.