"... With only 7 colors and a refresh time of about a minute ..."

Ouch, even worse than expected. The project is interesting, but I'm totally unimpressed by these numbers; I wouldn't pay a premium for a sub par display that is slow as a dead sloth. A traditional LED screen plus some tricks to save power would make a much better picture frame IMO. I would for example use a PIR/microwave sensor (they're cheap!) to detect when someone is approaching or stationing near the picture frame to bring the CPU back from sleep and turn on the display and backlight. It would never reach the same almost zero current draw of a epaper screen, but the quality gain paired with the lower cost would probably make it a viable alternative.