In this build I've reworked the custom map list, and added the following features:
Custom maps are now persisted between App updates, so you won't loose them anymore. They are even backed up to iTunes/iCloud if you're into that sort of thing.
Maps can be accessed from the iTunes File Sharing panel. Map files are ordinary png images (but with not-so-ordinary colors), so you can use iTunes to transfer maps to and from your device.
For sharing maps, there's an even better way. The map list automatically saves custom maps from the last five hosted games you've joined. They are labeled with a timestamp, and easily accessible for you to host or edit.
Note: As specified in the TOS, all user contributions, including maps, are licensed under Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.