It would be interesting to see the effect of adding some phone key pad protection material to the front faces. It may just be enough of a thermal insulator to stop the dew point occurring on the inside. Double glazing!
The procedure to remove mist on the windscreen of your "normal daily" car fitted with air con is to switch on air con and blow cold air. Cold air contains less water than hot air. If you blow hot air in a first time you increase the mist.
During driving a Morgan, in order to avoid mist on the instruments perhaps they have to be cooled.
I have the same problem on 2 instruments: oil and petrol in winter. "They all do that..."
Check also if there is no leakage at the T piece of the windscreen washer. This is close to the instruments and it can increase the humidity by splahing water all around. Generaly you see see the water falling close to the gearbox lever. An other "They all do that..."