I often do some seed dispersal of flowers along streets where I pass. So my choice is flower seeds. If you can access it you could also plant some fruits trees.

A field in legal limbo still has an owner, so the first move is asking who, not throwing. Then: native species, local ecotype, and nothing you can't name. Seed bombs suit tough pioneers (poppy, cornflower, yarrow) over anything that needs tending. Also worth finding out what's been sprayed there.