It should work, if you put it there before it gets to -75c. the board will keep itself warm, especially if it is in some enclosure. if the board has been frozen at -75c for some time, it could have trouble booting and would need some pre-heating. However, if you bring it in there, and turn it on while it is still warm, it should be able to keep working in -75c.
we could make a -75C tests of the RB800 in our temperature chamber, but it will take a few days for a result.