I am not a developer and certainly don't speak for the developers, so I can't answer your question about the future availability of any particular feature.
What I can say is that there are already tools out there that do what you wish. But they don't run on m0n0wall itself. They run on an always on box that polls the ethernet adapters of the devices you wish to track bandwidth usage on.
I run a program called IOG on an always on Linux box that does this. There are undoubtedly others, but IOG does this and only this, and does a decent job of it.
See:
http://www.dynw.com/iog/