Yeah, this is quite hairy to do in Unity in general.
You'd need a code-controlled camera, a way to set a texture size, a way to make a predictable capture area, make the camera draw to that texture, a renderer to draw the texture to the scene using the material/shader you want. It ends up being very project-specific and barely Spine-related.
The alternative of this, if the look is acceptable to you, is to leave a trail of clones of its mesh. But lowering the transparency may make it look a bit weird.
The "SkeletonGhost" component module that comes with the runtime will do this for you. Try it out.
Otherwise, another user has shared this tool: Tool for outlines, dissolves, transparency
It probably doesn't do exactly what you need but if you read the code, it may clue you in on how to get the render onto a texture. The source is hosted on github. https://github.com/aflesher/EntityPostProcessor2D