Nice idea IF we had such mirrors. Afaik the farthest away is on the moon.[0] Also a one-time pad needs to be as large or larger than the data used to encrypt it.[1] So your plan cannot be used for the several gigabytes wikileaks insurance file. But perhaps you can "pad" the key used to encrypt it? Probably.

[0] http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment [1] http://en.wikipedia.org/wiki/One-time_pad