There are ample posts about the problems users and developers experience with toolbars, RUI files, plug-in RUI files and their customisation. Instead of going on a rant about this (again), I would like you to consider to re-implement the system from the ground up, as the current system is too confusing and error-prone to try to fix.
I personally would not mind that this would introduce system that works but that is not backwards compatible with the current RUI system. Maybe a RUI converter would solve that problem going forward to V6.
Some of the problems I experience:
the toolbar editor is confusing to use. Really, this should be intuitive and easy.
there are multiple ways to achieve things, like adding or editing a button. They do not seem to have the same effect, sometimes the one way works, sometimes the other.
sometimes unable to add/change button icons; after a restart the changes are lost.
there is confusion over when toolbars are part of a plug-in and when they are in the default.rui file (do buttons get copied? Why? Why do I get “toolbar 00”, “toolbar 01”, etc.)
the toolbar layout gets fubar’d randomly. The only solution is
ResetToolbars(why is that command even present?), which forces uses to redo their customizations.
settings are stored not only in RUI files but also in the registry. When working with multiple Rhino instances, this gets confusing fast. As you seem to have solved this with the Rhino app and document settings for V6, why not also with the toolbars?
This list is by no means complete. I hope you will consider to build a new system for toolbars from the ground up. I would be happy to provide you with more input if needed.