Broad beans (aka fava beans). My absolute favourite biomass, nitrogen-fixer, as well as fresh and dried beans. The tops are tasty, too.
It also flowers very early in the spring, just in time to feed the first bumblebees. And the flowers smell yummy
Pampas is a noxious plant in NZ.
I'd get more (sterile) comfrey going as soon as possible.
Will alfalfa grow for you? It's a perennial with a heck of a root system, and harvesting the tops would shear off lots
underground.
Daikon: harvest slim ones, leave the rest till massive. Take the tops off with a machete and cover with
mulch to rot in the ground. Warning, if not thickly mulched, it can be a bit pongy....
One of the favourite biomass plants in Australasia is tagasaste or tree lucerne. Great stock/bee fodder, nitrogen-fixing, pioneer, fast growing, coppicing...
Downsides: it's not very good in the wind and it's pretty invasive in some places.