I just e-mailed Photon asking about the panel and usb chargers, their response: "LRI has received some solar panels samples that they're evaluating. They hope to have units for sale in the next couple of months. The USB chargers currently have no release date."
Based off of "Up to 20 charges from a D cell" I'm going to guesstimate that it takes ~500mAh to fully charge.
On another site, I found that it takes 2-4 hours to fully charge. So you'd need 125-250mA @ 1.5 volts. That's about a 3x4" panel.
The cheapo multi voltage harbor freight panels ($10) allegedly do 3v @ 200mA. In practice, charging one nimh, I get about 120mA sitting in my window, although this might not drop in voltage enough to be safe for the Rex.
I'm not sure if powerfilm makes panels smaller than 3 or 3.6 volts, but they do make quite an array of sizes, and for something this small, you should be able to find one for under $20. They're super thin and flexible, although not all of them have a UV protective coating on them, so make sure if you do order.