Hi, Pete, and welcome to Permies.
The first thing that comes to my mind is comfrey, the good old-fashioned stuff that spreads by seed as well as by extending its
roots. It's normally considered invasive because it competes so well, so many people buy expensive
root cuttings of specific cultivars that do not set viable seed. But I love the traditional stuff -- it competes well with grasses and spreads nicely. I started with a seedling in a tire in the middle of my orchard area, and now I have volunteers coming up everywhere and trying to crowd out the grass. (It probably helps that I scattered seed pods all over last year after it flowered.)
Many people report success with borage reseeding itself, but in my climate borage is fragile and doesn't do very well outside a well-tended bed.
Hope this helps!