It's not the medium that matters in creating saurergut, its the bugs. Using DME or LME to create a modest gravity wort works fine. Then throw in a handful of raw grain, cover the vessel to prevent any air ingress, and keep the wort in the 100F range.
Yes, that concoction may smell funky or bad for a few days. But as long as you don't allow any air ingress, the lacto species and other innocuous species will crowd out the stinkers and you'll likely end up with a nice multi-spectrum culture that produces good saurergut. The only bad thing is that these bugs don't survive well if you're not constantly keeping them fed. Breweries that use saurergut, have dedicated fermenters that are used to culture and ferment saurergut.