@miguipda: You found a very-very-very attractive price for so usefull device! Thank you! I knew about this charger from Lady_Ada (Adafruit Industries) some time ago (http://www.adafruit.com/products/390), but her device has a quite high pricetag, with your discovery it makes more sence! Please, keep researching this topic - it is so interesting to get solar power to whole bunch of routers with rechargeable batteries!
@derrij: Thank you for the link to solar panel! Very attractive price!
General comment: with solar panel and Battery Rider from SeeedStudio or set-up step-down regulator we will get double conversion - first we generate 5V and later we use it to power MR13U with its own power circuit to charge the built-in battery. To get super-optimal charging we need to get the battery out of the router and directly connect to battery contacts on Battery Rider, or maybe we can put this circuit inside MR13U, or maybe we can find some points inside MR13U to directly connect solar panel. Anyway you guys doing great job, keep researching!