Hmm... well in my experience (be it only from my own research) is that there are really only 2 ways to "pre-soften" the hair. What your aiming for is lifting the cuticle layer so that the dye can make its way into the shaft of the hair for the purpose of staining the hair. Besides chemicals to off set the pH of your hair, the only other thing is heat. So it depends on how natural you want to go with this process, personally I would try using a shampoo outside of the "good pH' balance which is normally 4.5-5 on the pH scale, if you can find a shampoo that is either more acidic or more basic that's going to left your hair cuticle and then using some form of heat, like a hot towel treatment right after using that shampoo and then applying your dye as quick as you can so that your hair cuticle does not close back up. Then condition as normal.
But just a word of caution, conditioner naturally closes that hair cuticle, so if your using your direct dye in conditioner, its kind of working against you at that point, even though I know a lot of people on here do that so help with the application process.
I might also try applying the hot towel when you have the direct dye on, wrap your hair in plastic and then put another hot towel on top of that to keep that cuticle lifted for maximum color pay off, or maybe using a hair dryer on a low setting to keep the hair as warm as possible. Hopefully my explanation makes sense! Good luck!