Haven't seen this before on the forums, and randomly came across, while doing some modding...
So, if you want to play custom battles with your minor nation, here is what you need to do:
1.Open patch.pack using Pack File Manager v1.1.
2.Go to the db-->faction_tables-->factions
3.Scroll down to the faction you want to play
4.Under the "Type" Section, change it from whatever it is, to "Playable"
5.Change the values for Early/Late Availiability to "True"
6.Save.
7.Play.