Before the recent changes to spine-c, I had an extension working to use my own texture atlas system by deriving from the AnimationLoader and RegionAttachment, and everything worked fine. (I derived from RegionAttachment instead of Attachment because there is an important typecast to RegionAttachment* in SkeletonJson_readSkeletonData.)
After the update, the RegionAttachment_init and deinit function are gone from extensions.h, suggesting that I shouldn't try to extend that object any longer. However, with that necessary typecast in SkeletonJson_readSkeletonData, I'm not sure what else to do.
I went ahead and extended it anyway, being careful to manage the memory myself. Everything works fine, though I had to add a check for null to the code you added in RegionAttachment_updateOffset that deals with self->region. (Since I'm using my own texture atlas, that's always null.)
I'm guessing that I'm not extending this the way I should be, so I'd love if you could give me some pointers. Thanks!