The tops can with stand cooler temps,but anything under 35* to 40* might due your plants in if any large amount of time passes at those temps.I think temps need to be above 40* but under 50* to achieve the goal your after,too much ground temps will want to keep the plant growing and needing fed.I have kept peppers in a greenhouse all winter with temps at and above 50*,but had problems with the plants becoming too leggy,I too thought they would go dormant and greenhouse would only allow 70% light.

As far as managing the dormant plants, they will all be kept completely in the dark with minimal watering. I figured I would have to do some pruning in late winter. They will have their own temperature sensor so as to keep them at a much lower temp than the seeds I'm trying to germinate.

The radiant under-bench heating system allows for some amazing control (when used with temperature controllers from Auber). I just don't know what the temperature degradation will be above the soil.

I think it will work, particularly if you cover the plants with frost blankets. IMO the key to greenhouse heating is to only heat the parts the need heating, which in your case is a very small area. By keeping the plants covered you can encourage dormancy and contain the heat. I do this too, only without the heat, and I can get lettuce through subzero winters.

The temperatures you are going for sound too warm for dormancy. 75F soil and 60F air is really warm for wintertime, so I might suggest something quite a bit cooler.