I broke down this year and bought heat mats, but previously I had some decent results by putting a heating pad under the tray. Just to be on the safe side I put the heating pad inside a Hefty bag to protect it from water spills.

Not sure how many candles the author was burning, but I expect that if you had to go thru enough candles to heat a GH it would not be that economical. Basically, no matter what you do you can't get more heat out of your fuel that is there in the first place. A heater can't be more than 100% efficient. What might work for keeping you warm if you put your hands near the burner won't necessarily work in heating a larger space like a GH. However, along the same ideas as this flower pot stove, you might want to research rocket stoves. They use a similar concept (with tin cans instead of flower pots), and can burn most any solid or liquid fuel. However the same physics restricts rocket stoves too; there is no magic here, if you need 5000BTU/hr to heat the GH, you need to burn enough fuel to produce that many BTUs. And with any combustion type stove, you need to be aware of proper safety when operating it in a small space, like a GH. Make sure there is adequate air for combustion and that exhaust fumes are not too concentrated in the GH (as well as general fire safety around stoves).

Turns out I have not much choice - I've got it heated to 50F because that seems to be the lowest my heater can be set without going down to pilot only which is too cold. All of my babies were okay this morning - I guess I'd better get back to work here so I can pay the big gas bill that's coming... many thanks Lori

I still think the only way this would work (aside from having the hot tub in the green house, would be to capture lost heat. Now my hot tub is very well insulated, especialy above the tub. My hot tub is a self contained tub, the heater is contained in the tub enclosure. But I think this method would work even better with an inground hot tub. Idealy, if the hot tub is not installed yet, you could build an inground hot tub (the heater is seperate with an inground hot tub). Install the just the heater inside the greenhouse. Otherwise, the place I have found the most Waste heat with my Tub, is above the heater. So hputting some sort of heat exchanger above the heater to capture the heat lost from the heater would work very well.