Now that sounds like an awesome plan! 🙂
A container object that can have multiple skeletons and a shared (keyable) draw order to allow all slots to go in front/behind/in between each other (ie an octopus whose arms could go in front of and behind other skeletons at the same time, or in-between the legs of other skeletons).....maybe something for Kickstarter #2 😉
Having something like that would be a godsend as it would negate the issue of having to do all this "slot management" in code at runtime, and make it a breeze for the animators to do complex multi-skeleton interactions inside the editor ready to played in-game easily with the "container" you mention.
It would also bring Spine closer in line with other non-gaming "animation only" software, meaning it might even open up a whole new market of users for you guys, and if anyone deserves as much financial success as possible it's you guys!