OK, so another update...
I received the chicken "nipples" last week, and have since rebuilt the plenum to work with them. Since these need to be installed on the bottom side of the plenum, I needed to rebuild the plenum. I have also reconstructed the housing so that it is now shorter in height and raised up so that the chickens can see and use the nipples hung underneath. I have done so, reinstalled the unit in the coup and with several low-temp days to prove it all, the whole is working very well indeed.
In fact, in sub-freezing temps the nipples themselves remain in working order and are warm to the touch. So, I think we can declare success, at least success of the new design working in low temps.
I will still need to see a couple of months of electricity bills though to see the actual damage there. So far at least, this is very encouraging.
Due to limited free time, I'll need a week or two to get the pictures of the new installation onto my web site. I'll place a short post to that affect when this has been done.
My thanks to jmagill for his suggestion to use the chicken nipple waterers, and of course to backyard chickens for the collaborative atmosphere where we could have this kind of discussion. That all solved the problem!