Thanks for letting me know! Sounds great 🙂
I had a quick look into it last week before posting. I guess the biggest issue would be the UNITY_EDITOR prepocessor directives, specifically if they are in the non-editor part of the library. as well as dissecting Editor code into a separate spine-unity-editor.dll. Application.IsEditor should help out here, no Idea if there are any performance repercussions for the current code.
For Unity Versioning having the same preprocessors as the engine (like UNITY_5) should help with compatibility, however I guess the library should also be linked against the unity version the user is using. Most of the time it is save to have it build against a different version (been doing this a lot), but sometimes it leads to a startup crash. I have some experience with this for our own code base which I groomed for the last few years, if you are interested in more pointers, let me know!
Was thinking to look more into the code myself, but I am not as versed int the details of your runtimes to dissect the code without breaking editor functionality.