Guaranteed official support doesn't help if Photoshop is lacking a feature you need. Try asking Adobe for a specific feature or even a bug fix and see how long it takes. The benefit of our small team is that we are much more agile and responsive.
Even if we were open to a source license, I don't think you really want to be working on the editor. The runtime source is open, which can facilitate development of many needs.
For the specific feature you mentioned, changing the texture used by a mesh so the mesh structure and FFD continues to be used, you could do this via the runtimes. Briefly, setup your mesh in Spine with a single texture. At runtime, get the MeshAttachment and adjust the UVs to point to a different region and optionally change the texture it uses. Lacking editor support to key this switch this makes it more difficult to key in animations. Events could be used to key the texture switches, though this would still lack visualization in the editor..