If I play Ambush, and then my personality comes home from the battle, is he now prevented from assigning again in the Attack Phase by the "No Second Battle" rule? (provided, obviously, that I find some way to unbow him after the first fight, or he was conqueror)
Ambush + "No Second Battle" rule
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
The "Second Battle Rule" is restricted to Attack Phases. Since Ambush doesn't occur during an Attack Phase, it doesn't apply here.
No Second Battle Rule: A Personality who has already been in an attacking army during a battle resolution in the current Attack Phase will not move to a battlefield.
(Emphasis mine)