Hey Leo,
It is an interesting idea, and perhaps it could be done. I would definitely be more inclined to do the article for $25 and 50%. I do wonder if there is an issue with figuring out payments and such. I mean, if you work with even 5 or 10 writers it could be difficult to verify payment and keep track of who wrote what.
Now, if Medium could add in a feature for this where it was tracked internally you would really be on to something!
Personally, I am on Medium to write articles that I enjoy writing and to build a personal brand/following. I think this is tough to do if another person puts their name on my article. It could be a good model for some, just not for me.