I'm not sure I've a full understanding of what this bus is all about,
but I can't see
why it can't fit inside KMS, with maybe a V4L bolted on. The whole
point of KMS is to provide a consistent userspace
interface for describing the graphics hardware in enough detail that
userspace can use it, but without giving it all the gorey details.
So we've reduced the interface to crtc/encoder/connectors as the base
level objects at the interface, internally drivers can and do have
extra layers, but usually no need to show this to userspace.
KMS at the moment doesn't really handle dynamic hotplug of new crtcs
connectors etc, but I'm not sure that is needed here.
It sounds like you just have some embedded building blocks you want to
put together on a design by design basis, please correct me if I'm