I think the element of surprise is beyond the scope of the Mouse Guard scene-framing style.
If one wanted to account of it, however, I would do it as some kind of gear that gives bonuses to Attack actions on the first exchange. Or something similar.
Today, I treated it as an advantage die for turn 1… My 9yo brought a group of 10 rebels (with a Rebel Wise roll) at the party… The pile-o-Rewards makes a big difference… But the rest of the party had set it up as an ambush. Result: 4 injured Guardsmice, 10 dead rebels, and two Guardsmice down down 1 Nature.
A 1d advantage on turn 1 really does make a big difference, but not an overwhelming one.