I'm also looking for an almost-photo-quality double-sided paper that doesn't cost the better part of a paycheck.
The closest I've found is the HP Brochure and Flyer Paper, but when doing dark solids it swells and buckles too much.
I would even settle for a true photo paper with one coated side and a plain back that would render a line-art calendar. I've tried a few photo papers, but the reverse side always seems to have the logo, or else it will not accept ink at all.
I would think there would be a demand for things like this.