To keep the light coherent and useful, a mirror would have to be flat- perfectly so, since even the slightest imperfections in the reflective surface would scatter light, including the glass itself if the mirror is made of silvered glass, since laser light would pass through the glass and be altered according to the properties and impurities of the glass, hit the silvering, which would also alter or scatter the light, and be reflected back through glass, further altering the light. How much 'power' the beam would retain through that I don't know, but it would be diminished considerably, I would think. A concave or convex surface would be able to focus the light but for the most part it would be scattered and useless, except perhaps at the focus point. It might be hard to get the plane to fly where the focus point is...
Interesting question though. Maybe the laser would burn through the imperfections in the silvering and glass and fry everything behind it. More likely, someone in another plane would spot that big glistening mirror and take it out conventionally.
Maybe Radio Astronomer's closer to the technology than I've been, or knows someone who is. I've only taken one course and that was back when people were still giggling whenever someone mentioned lasers, much less asked about what effect mirrors have on them.