Heating in Maine

Let me ask a couple of questions before I make a complete fool of myself.
-You say "massive" concrete slab. How thick is it?
-What is the fuel source of the heat.
-What is the output (Btu or watts) of the heating system.
-What is the construction of the walls/roof.
Now I'll do what I was going to avoid (potentially making a fool of myself). I believe the problem is that the cedar deck is blocking the heat. Since your heat relies on radiation from the slab, the deck blocks that radiation. Have you ever taken the temp below the deck in winter?

(another) Mike

 

Another questions.
Is the slab at ground level, or is the deck at ground level? If the slab is at ground level, it needs to be well-insulated with several inches of rigid polystyrene, to keep the heat inside, obviously. Even if the slab is below grade, it should have some insulation. (I'm not sure about the building code requirement for Maine, but I suspect the foundation should be several feet below grade to be below the frost line)
You are right in one respect-a heated slab will be slow to respond to the drop in temperature a greenhouse experiences when it loses sunlight. The thermostat should be adjusted to have sufficent lead to help anticipate the need for heat. (Many thermostats are adjustable to call for heat at a given differential above the set point, and turn off at a differential from the set point)
One of the principles of a heated slab is that some heat is provided by infrared radiation which heats the objects they fall on. Your intervening deck obstructs that radiation from reaching your plants (although I suspect the deck is very comfortable). Although you don't mention it, it is possible that there is not enough space in the deck for convection currents. My thought is that you would benefit by pressurizing the "crawl space" with a fan and duct arrangement that would force warm air through the space between the deck boards. I still believe that a big problem is that the deck blocks the heat from the slab. Another thing that might help is to have a minimum flow setting to never completely shut off the hot water flow to keep the slab warm (maybe it already does).
All that being said, the problem is not hot water heating per se, but that you may not be able to provide heat at an initial rate that matches your heat loss in some conditions. An alternative of last resort might be to replace the in-slab piping with above-deck fin-tube piping like traditional greenhouse heating. This is obviously all theoretical, since I haven't seen your greenhouse, but I hope I have been of some help.

(another) Mike