Mike,
I tried the fan blowing air onto the back of the mirror and up the tube last year at astrofest and it made no difference - you are still introducing wet air into the tube and if anything the fan is going increase the rate of heat loss from the mirror. I think the only way to stop this is to keep the mirror above the dew point temperature.
I'm going to try a Kendrick primary mirror pad with an inside/outside temperature probe to measure the temperature differential. It would also be possible to use the Digifire 10 controller that has this temperature measurement and control functionality built in however I've already got a Thousand Oaks unit.
http://www.kendrickastro.com/astro/newtonian.html
This is the same principle that is employed in the Kendrick Dewguard secondary mirror heaters that are installed on quite a few SDM's. These are programmed to keep the mirror 1-3 degs above ambient.
I reckon dew formation is going to have a far worse impact on the scopes performance than any slight current that is generated. I've had the 2024 10" Primary Mirror Heater and DG1 secondary heater on order for a few weeks.
Peter