Maybe house the rear of the instruments in a form of insulation the metal bodies will attract the damp, you could even consider running 12v trace heating around the instruments.
If the instruments are hermetically sealed no moisture will enter.
It won't stop condensation on the outside but they would be dry internally if opened up dried in a low temp oven then sealed.
It should not be that difficult to achieve how many modern motorbike instruments steam up?
oooh maybe another project for the list of things to check/do
A quick look at the back of my speedo shows several not obviously sealed moisture entry points
see red lines
