So, for what it's worth, I've done a ton of reading on what various enzymes do to break down starches and dextrins, as well as the process of malting and kilning and how that converts sugars, both fermentable and unfermentable. I now realize Carapils is very different from Crystal malts - somehow (it's proprietary), Carapils ends up with a bunch of "enzyme-resistant dextrins" that cannot be broken down in the mash... but I'm not sure why. Like marticaivxavier said, Crystals do have fermentables, with progressively fewer as you get darker (having to do with the higher kilning temps).
Now I'm starting to look into whether there's a way of adding extra debranching enzymes to a mash, as I have a feeling that would enable the breakdown of certain dextrins that branch off the long glucose chains. Perhaps Carapils is full of short branching dextrins, which could become fermentable if mashed with the right enzymes. But at this point, it's more about curiosity than actually wanting to be able to create fermentables out of a dextrin malt.
As a sidenote, Brewing Science and Practice (2004 book, Dennis Briggs et al) has a ton of great information for anyone of the scientific persuasion, that doesn't mind a lot of chemistry and biochemistry. I just found it this week and have barely cracked it. Thick, but well worth skimming if you want to truly understand the science in the brew.