If I'm understanding your question, you are asking if you can keep a full, large stove burning at a low temperatures & the answer is absolutely, providing that your stove is air tight or close to it. Unlike wood, a coal fire's heat is not totally dependent on the amount of coal you have burning but rather by the amount of air you provide it. (assuming at least a minimum amount of coal in a deep bed of course) A leaky stove does not give you this control, but a good tight one does. I burn my Harman TLC2000 regularly at (stick on) thermometer temps of 120*-140* with ease when it is not real cold out.
At these low temps you'll burn allot less coal & have to service the appliance less often. Keep in mind that when you are burning very low fires, you also will have less chimney draft & are really working at the edge of your appliances working envelope. It's easy to lose a fire like this, but experience helps allot.(also one of these helps http://www.4thegrill.com/mavet73.html
This link is broken, either the page no longer exists or there is some other issue like a typo.
It remotely alarms if your stove gets to hot or gets to cold)