Hey Duntov,
Much like awake', I claim no subject matter specialty on fridges and humidity.... I do however have 6 cheese fridges in the basement cave that I maintain and can share my experiences with... They are Danby silhouette models or 45-65 bottle type wine coolers style fridges. That hopefully paints a picture to what I've got.
My humidity seems very stable in the low to mid 80s, simply by keeping the fridge full of cheese. Being waxed or maturing boxes (with holes). Plastic wrapped cheeses of course are in a separate fridge ... Don't care about humidity there.
I used to use ultrasonic humidifiers when the fridges were half full. With the dryer winter coming I will have to probably put the humidifier back in ... And I also use a DC fan that is connected to the same timer as the humidifier set to 1/2 hr on 1/2 hr off. Seems to do the trick and only have to refill once ever 2-3 days... So quite manageable. I've also toyed with the idea of externally fed water to the humidifier bu haven't come up with a plan yet..
When reading your post, I had a concern about the runoff water being used to feed the humidifier. I use bottled spring water to fill the humidifier ...,used to use distilled water.. With the idea of clean and filtered water not clogging up the micro components providing the misting action of the ultrasonic humidifier. And clean water vapor on my cheese. I fear there would be micro particles and waterborne molds and mildews eventually as the fridge ages with your idea of using the drippings off the condenser and drip tray. Ether creating a clogged humidifier, or worse, contamination of some sort. I have no science to support this... Just paranoid thinking. ..lol
Good luck moving forward..
F