Pretty easy to get Pips to do what you want, from what I can see, though I will admit it does use some space. It looks like they plant anywhere there's exposed cultivatable soil, you just need to dig a natural path for them with that in mind (keep dirt as the top layer and they'll eventually plant on it). Using the material overlay helps a lot with this. Then put a storage bin with whatever seeds you want them to use on the other side of a door leading to the space, and viola!