A player can play an Outer Walls for its battle ability in an Ambush, as mentioned by Loïc here. But does it count for the Ring of the Void trait? I'd loved you say "yes" because I play enlightment. But I think it's not a successful action (no change in the game state occures).
Successful A successful action or other occurrence is one whose effects produced one or more actual changes in the game state. Example: An Action card being discarded, an Event leaving its Province, or the fact that an ability on a Personality has been used, all do not count toward success. Effects that are negated, that do not occur, or that had a one-time effect but failed to affect anything, are not actual changes in the game state.
Ambush Limited: Target your unbowed Personality: Create a battlefield (not at any province). Assign your Personality's unit to attack there. Assign another player’s target unit to defend there, even if its Personality is bowed. Other units will not move there. Fight a battle there (after this action's resolution). After the battle ends, lose 5 Honor.
Outer Walls Battle: Even if you control no units at the current battlefield: Give its province +3 strength.
Ring of the Void After you successfully resolve your fourth or later non-Kiho action from Strategies in one phase: You may put his card into play from your hand.
EDIT: ignoring the new MRP which is not effective right now.