EDIT: I'm actually not sure if the unit in question is a sovereign, but it has a lot of traits just like sovereigns have.
I was able to recruit the enemy sovereign, Lord Blackstone, as he was in an army with a recruitable champion.
Also, recruiting doesn't seem to cost anything - I don understand why I wouldn't run around recruiting everything I can?
debug.err, screenshot and savegame to be found at
http://linque.pp.fi/Elemental/RecruitBug/
