It's been a while, but I do have one new update to report on this build out...
I've now fermented out a pair of good ales and a lager in this chamber, and it performed admirably where all that was needed was cooling.
I've got a robust porter in there right now, and it needs to ferment a little warmer than my basement temps right now. So, I picked up a reptile heater and plugged it into an old lamp I had hanging around, then plugged that into the hot side of my ebay temp controller.
I'm pleased to report that this seems to be working perfectly! I had let the fermenting beer sit, not realizing that the temps were too cool. It had settled down to about 62 or 63, when the yeast I used called for 68-73. Once the heating element was introduced, it took a couple hours to warm the fermenter up sufficiently. I believe I did overshoot the temps a bit and the fridge kicked in as designed. I'm not sure exactly how much the two overshot before finding equilibrium (I didn't really feel like sitting down and watching my chamber for hours last night!), but when I checked this morning it was happily sitting at exactly my configured temperature. I've got it set to a tolerance of .5C, and it's happily sitting right in there, with no apparent tug-of-war between heating and cooling elements.
I am quite pleased.