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.
A player, 5 June 2008
Sneak Attack Reaction: After engaging in a battle: The Attacker has the first opportunity to take a Battle action or pass during this battle. Play then proceeds in turn order from him.
Insurmountable Obstacle Terrain Battle: After each time an action moves a Personality home from this battlefield, dishonor him. You may take an additional Battle action after playing this Terrain.
Untested Scouts Recon Limited: Target a Province. Until the turn ends, after engaging in a battle at the province's battlefield, you may take one additional Battle action to play a Terrain from your hand.
Removing the Advantage Reaction: After engaging in a battle, if you control a unit at the current battlefield: Play a Terrain card as an additional Battle action.
If I declare an attack, assign a unit and use one of the two legal methods for playing a Terrain "sneak-style", can I then play Sneak Attack for a total of two Battle Actions before the defender's first chance to act (one from Sneak and one from Insurmountable Obstacle's granted additional Battle action)?
Brook Cunningham (rules official), 5 June 2008
If I declare an attack, assign a unit and use one of the two legal methods for playing a Terrain "sneak-style", can I then play Sneak Attack for a total of two Battle Actions before the defender's first chance to act (one from Sneak and one from Insurmountable Obstacle's granted additional Battle action)?
Well, technically, you'd get three Battle actions before the defender's first action in that scenario, counting your playing of Insurmountable Obstacle in the first place.
But, yes, that's all quite correct:
Upon engaging, the delayed effect of Untested Scouts' Recon action would trigger, allowing you to play Insurmountable Obstacle, which gives you another additional action immediately after it resolves, and after all that's done, we move down the A-F "timing tree" (see here) from D, delayed effects, to F, Reactions (such as your Sneak Attack). Ouch.
Almost the same scenario if you're using Removing the Advantage. Upon engaging, you can React with that, allowing you to play Insurmountable Obstacle, which gives you another additional action immediately after it resolves, and after all that's done, your opponent can React to engaging. Assuming the poor soul passes, you can then take another Reaction to engaging (such as your Sneak Attack). Still ouch.