For those interested, I give some experience with naturalized brassica species:
At one site, 3 different brassicas have been naturalized together for at least four years without crossing, so far as I can tell. These are Red Russian kale (B.Napus), Giant Red Mustard (B.Juncea), and common field mustard self-introduced as a weed. Presumably this latter is B.Nigra. I have not attemted to naturalize a B.Olereaca there but it would be a useful extension of the experiment to try a well-adapted olereaca kale. My suspicion is that Olereaca cultivars in general do not do as well in difficult circumstances, at least in this climate with warm buggy summers and moderate winters. In a much colder climate they would likely prevail.
In any case, the gene pools of Napus, Juncea, and Nigra seem to be very stable and isolated for practical purposes. I bring up this topic for the more average gardener to take advantage of the ease of saving seed from Brassica and point out that one can easily save seed from all the common groups simultaneously presuming the limitation of one cultivar per species.