Earthworm castings are pretty high in nitrogen. It would have to be apretty thin layer or soaked somehow. It is a pity that they dont hold their shape in water, they look better then aquasoil fresh.
This is something I find interesting and puzzling...would it work as a first layer or would it be to rich in organic matter?
That was my first instinct as well - but farm animal manure, is basically pure waste. When you think about what worms are eating (bits of organics they find mixed in with soil) they are using up organic matter (as they add body mass it has to come from somewhere) but some of the soil is also passing through them, they are essentially further composting the soil they are living in unless I misunderstand things.Cow, horse, and pig manures contain a lot of organics and are dangerous to use in a NPT or anywhere else. I suspect that earthworm manure ("castings") are similar.