Gah, posted at the same time as Pharan. Here it is anyway:
What would you want to customize? Skeleton does only a few things: 1) stores data, 2) knows how to update world transforms for bones in the right order, and 3) knows how to apply constraints in the right order. I think it makes more sense to use composition, as SkeletonAnimation does. What I mean is, instead of using inheritance, SkeletonAnimation has a skeleton instance it uses to do things (apply animations). It makes sense to subclass SkeletonAnimation to add extra functionality.
To follow up on Pharan's post, spine-c does its own sort of object oriented programming, but you don't really need or want to use that in this case.
