I was thinking about this today and if I'm not totally lost, a simple, mini-usb chargeable external battery TOH could be made for example with these (the phone gets about 8-10 mm thicker from the bottom half):

https://www.sparkfun.com/products/8483
http://www.prodctodc.com/ultra-small...l#.UtVSLLS-FCk

The output usb-port should be replaced with some wire for connecting to the Jolla's 5 V + GND pogo pins. There should be also some protection (voltage cutoff circuit) so that the phone does not draw the LiPo empty. However, I'm not electrical engineer so this is just an idea

Edit: this is maybe better: http://www.adafruit.com/products/259#Description but it does not ouput 5 V
Edit2: all-in-one solution https://www.sparkfun.com/products/11231 Maybe I should do this?