Recently I played around with 3D coat and it has a nice live sync with external image editors for textures.
A shortcut like "open in external editor" in Spine that simply opens the currently selected image in the configured editor might be useful for quick adjustments (no bone transformations applied).
An advanced version would probably have to open the current skeleton pose as a PSD composition of smart objects to keep the original files intact. Might be useful to create variants of attachments directly on the current character pose - no screenshots or exports needed.
Regarding the license of the .psd format I couldn't find any news either, but they seem to be open for 3rd parties.
