I'm at best a dabbling modder, so take what I say with a grain of salt.
I would guess you'd need the [CULTIVATED:<region(s)>] tag to get things to be offered by villages, but it may well be that actual seeds are required as well, and might also require that it's edible, as that's currently the only thing UrW does know about regarding plants. Also note that you can specify plants to grow in settlement and/or meadow tiles, for instance to get them to behave like e.g. nettles and clayweed.
I wouldn't use an existing mushroom for your purpose, but add my own, which seems simple enough (provided you're more talented than I am at graphics, as you'd want a distinct tile gfx for it, but I guess you could copy and hack an existing one).