Best I've seen for bio char has been bamboo. It's a bit shit as a fuel though.
I planted a lot of ice cream bean for the nitrogen fixing & mulch about a year ago. They really didn't grown as quickly as I had expected. I think once they're established they'd grow almost as quick as black locust but without the spikes. They don't like frosts though, so not great if you get those.
My elderberry is due for a prune so I'll be planting cuttings from that. I was surprised by how quickly that grew in a single season. I think elderberry stems were used as straws back in the day.