If you turn off all toolbars in for example Part Design workbench and set TabBar to top position and lock it. This is basically the end result you are after? I know not all tool buttons are supported ATM and some toolbars produce issues. But still from the concept point of view that is basically it?I would love to have the option to place it above the toolbar (or have the general Preferences option to use the "default toolbuttons + workbench selector" or your new TabBar system...
Anyway it turned out tabs on main window level are not as straightforward task as one would expect. Majority of things i tried out didn't work good enough. There are basically two approaches left that have potential but i can't be sure before investing substantial amount of time in it if it will work out. As for workbench buttons on TabBar widget level that i guess is possible but investing substantial amount of time will be needed to make it manageable.
Anyway current plan still is to support both use cases. Therefore what will happen next is i will build up code base by adding mentioned features to it and some other features i plan to do. After turn that in release material and release it in pieces as the code is cleaned up. I have done it like that with the current released code base and the procedure worked out good.