I was thinking of doing the same thing for my dob. Regarding being too cold, the key is, as you said, to match the temperature of the scope to the expected air temperature at dusk. I was going to make a new cover for the top and bottom of the mirror box, and run something like dryer duct. Input would be bottom, so the cold air would hit the back fo the mirror first, in case of the cold air being too cold and causing dew.
However, I dont' think condensation will be a problem during the cooling cycle. Any condensation will occur on the iceblock or refrigerator coils in the cooler, pumping saturated air, at worst, into the dryer duct. But the air would pick up heat along the way, drying it out. By the time it hit the hot mirror, it would not be able to deposit condensate on it. Condensation occurs when dap air hits something colder than the air. The coldest thing in the system will be the block of ice in the cooler, so no problem. I was planning on drilling a couple small drain holes in the cooler to let the water out, and hopefully prevent the air from becoming even more saturated.
The risk for condensation happens when you disconnect the cooler and expose the cooled parts to ambient conditions. But if your thermostat has worked properly, your mirror wil be at or slightly above ambient, which is the goal of fans to begin with.
It seems to me the biggest thermal problem with mirrors is the big chunk of glass in the middle. Fans can only cool the surface to ambient. If the ambient keeps falling all night, you have a hard time catching up, and there will always be a big gradient between the mirror core and surface. If you can start with a mirror uniformly cooled throughout to the temperature you will start observing at, it should make thermal equilibrium easier throughout the night as temps fall.
I did have a backup plan, though, incase the primary does dew over. I always bring some chemical hand warmer packs with me, because ti gets pretty darn cold here on a clear night, even in August. I could put these in the cooler in place fo the ice to warm up the mirror a bit to get the dew off. I am also working on boundary layer fans, which will include a few watts of resistors for the same situation. From what I've been reading, I shouldn't need them, but resistors are cheap, and trips to dark sky sites are rare and valuable to me.