Unfortunately there's not a way to do this in the editor. The way a Spine skeleton is rendered in all game toolkits results in multiple overlapping images. It takes work specific to each game toolkit to render to a temporary buffer, then render that with transparency.
You could use an event to control the runtime effect. Or you could use a specific slot's alpha to control the runtime effect. For example, you might choose a large attachment, like the torso, so you can see the transparency in the editor, even though it's not the entire character. Then at runtime you would render the whole character using the transparency from that slot.
