I almost hate to go down this road since it's all hypothetical at this point, but why do you say there is no bonus to me doing it for them?
Here's the thing, self publishing is not easy to do right, and it's very, very, very easy to do wrong. The same goes for publishing in general.
It is the easiest thing in the world to find a publisher to take your book and publish it for you. Easy, easy, easy. Anyone can do it, and many people do. If you lower your bar far enough, you'll find a "publisher." Finding a good publisher is not so easy (well, actually, finding them is really easy, getting them to take your book is generally the challenge).
New presses sprout up every time a butterfly sneezes, and in particular author-started publishers are especially common. Many are well meaning. Some want to help other authors find a home for their work. Others want to add credibility to their writing by surrounding themselves with other authors as a way to make it appear like their work isn't self-published.
If all parties are informed, I say go for it. But if they're truly informed, it makes no sense not to publish under their own name/own publishing house, and just work together. As you pointed out, it's almost free with Spark. It is free with Createspace. Why not just work together to market titles without taking the rights away from someone?
An example of that kind of co-operative would be
http://emblazoners.com/ - Started by Susan Kay Quinn (I hope I spelled her name right), the Emblazoners are a group of self-published MG writers who work together to market their work. Everyone keeps their rights. That has some value. (ETA: to clarify, I'm not part of this group, I'm just familiar with them as I've seen some of their co-operative efforts).
But a publisher that opens their doors without first having years of experience in the trade industry - working with established, successful presses, learning the ropes (acquisition/editing/production/marketing...etc etc), has zero to offer. They will only hurt the authors they "publish." I have yet to see any exception to that. And if you take a few weeks and scroll through the BR&BC thread here on AW, you'll read some truly heart-breaking stories of authors whose books have been destroyed by such endeavors.
I know it was all hypothetical to you, and likely a moot point. On the surface it looks like it could be a good idea. But without exception it isn't. Truly.