Hi William,
I know there was a user (Joel) who was looking at using an Android e-reader and think it worked for him. All I can say is that if it runs Android and has access to the Google Play Store, there is a good chance it will work, but I can't guarantee it. Hopefully Joel is still active and using this and can give some feedback. Anyone else have experience of this? There was a post on the forum (search e ink).
Backing tracks can be stored anywhere, however, newer versions of Android are more rigorous in granting (and more importantly continuing to grant) permissions to files that the user selects in an app. When you choose a file, OpenSongApp requests permission to view/access the file and tries to keep a persistent permission. However, Android can revoke these permissions after a while, but will certainly do this if you uninstall the app, or clear the app cache. Updating the app, rebooting the device, etc. does not revoke the permission, but an uninstall/reinstall will.
To avoid this, the app asks permission for a single OpenSong folder on first run. This gives access to every file it contains (even in the Media subfolder) as long as you use the app. For this reason, storing backing tracks in the OpenSong/Media folder is still the safest option to guarantee the permission isn't lost for all of your files.
Hope this helps,
Gareth