Hi Cheapskate,
Welcome to the forum.
If you haven't already checked it out, make sure to research Gary Reysa's website
www.builditsolar.com. Since he lives in the NW states, he knows what snow is.
Speaking of which, you don't happen to live in that particular city at 52 degrees N where it was snowing last Tuesday, do you?
Before deciding to pour a complicated concrete slab, have you considered heating the extension purely by air heating in those collectors? The 3-4 window panes that you have sound like they can do the job, but I'm basing my guess on Gary's air-heater collectors projects. The concrete slab in your extension is a heat sink that will help moderate the temps inside, even if it's heated only by the air above.
If you do decide to heat the slab with a glycol loop, then for efficiency's sake you'll be looking at insulation underneath it, and around the exterior wall surfaces, to keep that heat from conducting away.
It sounds simple, but I suspect that running the pump directly from a solar panel could have problems. For instance, the pump may shut off as a cloud passes, but the air cells are still warm. When the sun comes back out, the collectors start cold. While the glycol isn't circulating, the heat they've collected is just radiated away from the collector. In partial sun, the collector may continue to warm up while the pump can't start. If the pump's solar panel is shaded by a tree or someone parks their van in the wrong spot, the collectors could be hot for a long time.
But with careful design, it does sound practical. There's a building heat loss calculator on Gary's site. If you look into the glycol heating collector a little more, perhaps from a scientific perspective, you may find some research or analysis that will help you guess the sizes of all the components.