Preserving sweetness during baking without sugar
I hate answering my own question, but after a month I did (finally) find a pseudo-answer      that works for Stevia.  I've not yet tried it for any other sweeteners.

Melt into fat and freeze it

For example, the recipe I use for cookies needs four (4) tablespoons of melted butter.  I started by melting two tablespoons in butter (well, 1TB butter and 1TB cocoa butter but semantics aside...), and mixed half a tablespoon of stevia into the melted butter.

Then I poured the Stevia-butter mix into molds and froze until it was solid.

I proceeded with the rest of the recipe as normal, but after everything was mixed, I took the frozen Stevia-butter, chopped it into very tiny shavings/pieces, and mixed it into the batter.

Poured the cookies on the tray, and baked in the oven.  Result: nice, sweet, almost zero carb (~0.05g) cookies!!!

My working theory

I'm not confident in my theory, but I think the fats - especially saturated fats - will protect the Stevia early during the baking process. Couple that with Newton's law of cooling (ie, although the batter is at room temperature the sweetener is at sub-freezing temps, and will therefore take longer to heat up than the rest of the batter).  While the batter cooks as normal, the fats will melt into the batter, revealing and mixing the sweetener with it.  This minimizes the time the sweetener is exposed to the full-force heat of the oven, which prevents it from degrading under heat.

This may not be the best process for fluffy or flaky goods like breads, but for dense, sweet goods like cookies, muffins, brownies, etc, it works well. 

Yet again, that's just a working theory....