Hello, while you can use PaintTool SAI to create PSDs structured using the PhotoshopToSpine tags, and then export the file using Photoshop, from what I could find PaintTool SAI doesn't offer scripting tools and hasn't been updated since 2016, so I'm afraid creating a dedicated PaintTool SAI export script won't be possible.

https://www.systemax.jp/en/sai/history_v1.txt

If this changes or if I missed some info, feel free to let us know!

Photoshop is terrible. I think we are on the same page with that! Unfortunately it's widely used and has scripting support. It's not that we don't want to support other apps, it's just not always possible. SAI doesn't appear to allow writing scripts/plugins.

The same is true for other apps, like Clip Studio Paint. We use that and would love to support it, but they don't have scripts/plugins and they aren't interesting in putting in Spine support themselves.

Affinity Designer also doesn't scripts/plugins, but they are awesome and added Spine support themselves. Unfortunately it's not as powerful as the Photoshop script. We talked with them about improving their support, but it's a lot more complex than the basics they did and they understandably don't want their time sucked away fixing bugs in their implementation. They didn't actually say no, but they haven't done it yet either. Anyway, Affinity Designer could be a somewhat less powerful option. If they had full support like the Photoshop script, we would happily push everyone to use Affinity Designer.

Another option is to use Photoshop CS2. Adobe made that free a while ago, or something like that, and it is very easy to find the official, unprotected software, no crack needed. It's super old and crappy, but it can run the PhotoshopToSpine script. If you can export to PSD from SAI, you could use CS2 to process the PSD into files Spine can consume, giving you all the PhotoshopToSpine script features.

We've discussed many times a way of supporting exports from any image editing software. Most software can export layers as PNG files and we could have Spine process [tags] in the PNG file names, but unfortunately the layer folder hierarchy is lost. That makes it a lot less useful.

You might want to conciser importing from PSD files directly in spine, rather than spending time making scripts for other apps.
For example we made some scripts in Unity that allow us to read layers / groups directly from a PSD and build our own custom atlas. Although there might be some limitations I am not aware of.

Having Spine processing PSDs is not great because they can contain layers that effectively only Photoshop can process. It doesn't feel great to only partially support a format. However, you are right it would work fine for PSDs that have only simple layers that are already rasterized, which is likely what PSDs that non-Photoshop tools like SAI would create. I agree supporting that is worthwhile. I don't look forward to the support issues that will certainly arise from people using Photoshop and having Spine process their PSDs. Those people should continue using the PhotoshopToSpine script so they can use all Photoshop features.

I've updated the issue to track this task:
https://github.com/EsotericSoftware/spine-editor/issues/476