I used mappero in offline mode successfully while rafting this summer (for 10 days, using external battery pack called AcmePower UC-5 and custom-soldered USB cable with middle wires shortened to recharge). It was in Karelian taiga and GSM coverage was, well, not very dense I used python scripts written by vadp to upload maps that were created for Ozi Explorer. I think it must be possible to pre-cache some data from say Google maps, but I didn't try it myself.