A product which I would love to see, but which I do not believe exists is an RC paper, which has a back side which can be printed on by an ink-jet printer. I wanted this a while back for a project where I wanted to make a calendar with RC prints, and print the calendar grid on the back, but this material would also work for making your own postcard stock. A few years back when I inquired, it seemed that the people who make silver based emulsion were clueless about what properties it would take to make the paper compatible with inkjet printers, and the inkjet people are not aware that silver photo emulsion even exists.
The solution that I ended up using was to print the calendar grids on 8 1/2 x 11 label stock, stick that on the back of the print, and then trim....