I know it's pretty normal to have some lumbar flexion at the bottom of a pistol, but if I had disc issues, I would try to prevent that (disclaimer: I'm not a medical professional of any kind). I would either hold onto a stationary object (post, counter, low hanging rings, etc), or do a goblet pistol squat. Even with light weight you could hold it farther out in front of you to help keep your back straight. Holding onto an object might be simplest, as stationary objects are easier to find than 10-25 pound objects that can easily be held at arm's length, but if your pistol game is good already you might want to add weight anyways. I would also probably not quite go totally a#@ to grass like a standard pistol.