Sad that this didn't happen so far. This way we could have created our whole worlds with B3D instead of using own formats to reparse it into B3D afterwards.
This has been possible from day 1. There's nothing stopping you using the .b3d format and creating your own custom nodes.
What hasn't happened is for some type of standard to be drawn up.
I currently use 2 custom nodes for my app :
SEQS - for animation sequence info
PHYS - for ragdoll/physics joint settings.