Silicone rubber cures by water vapor diffusing into the un-cured material. The problem with curing below freezing is that the humidity will be very low. You can heat the part with lamps, but it will still be a very slow cure since there won't be much moisture in the air.
Many RTVs suggest letting the material 'skin' before assembly, so if the one you're using does suggest this it might help would be to take the water pump indoors, coat one side with RTV, and allow it to 'skin' inside before taking it out and installing it. The engine block side might just have to cure best it can, but whatever the case I would allow much longer than usual to see if it cures.