Does ornamental cabbage make decent sauerkraut?
Apparently, it has been done. According to this website, the flavor will be strong compared to sauerkraut:


  The flavor is strong; hard to describe — not just simply more acidic but strong, and the texture is a little tough... I also find that kale ferments accentuate the salty flavor, no matter how carefully I salt and I have no idea why. Mirror this with bold flavors. Ferment kale with lots of garlic, chiles, smoked salts, chipotle, curry or other unflinching spices.


So the proposed solution is to make kale kimchi mixed with other vegetables, or add kale to a normal sauerkraut to dilute the harsh flavor.