Interesting thought Matthew,
Some relevant stuff here
I am not sure how Mercury behaves during its freezing process - in terms of expansion and distortion, but also in terms of any crystallising and how its reflectivity changes.
These properties might make it difficult.
Keeping moisture vapour away at -39C might be difficult !
Not to mention the ongoing costs costs of a cryo system.
It does evaporate into space though, so putting one at the L points wont work either.
http://forum.nasaspaceflight.com/index.php?topic=7968.0
Seems they have a new option that is a spinning liquid of molten ionic salt but still near room temp that is Silver coated
http://science.nasa.gov/science-news..._liquidmirror/
Rally