Is there an additive that can lower the freezing point of jet fuel?
There is an additive known as a Fuel System Icing Inhibitor (FSII), it is designed to be mixed with the fuel of non-fuel heated aircraft i.e military. Mixed in at about 0.10-0.15% is optimal to reduce the formation of ice crystals, which will stop it freezing.

Source: http://www.advancepetro.com/fsii.htm

Alternatively, Jet A-1 fuel has a freezing point of -47°C, which is below your threshold and could be useful. Or if Jet A-1 is unavailable you could try Jet B which has a lower freezing point of -60°C, but is more dangerous to handle, and is less prevalent and hence unavailable at many locations.

Sources: http://webarchive.nationalarchives.gov.uk/20100814170713/http%3A//www.dstan.mod.uk/standards/defstans/91/091/00000600.pdf

http://www.csgnetwork.com/jetfuel.html