> This is my plea to the Emacs gurus: Implement a suitable area at the top of
 > windows/frames/whatever to pave the way for a serious tab bar implementation.

A side window at the top of a frame (see section 28.19 Side Windows of
the Elisp manual) is such an area.  A couple of years ago I wrote a
package called frame-tabs.el which uses it.  I attached a copy of it
so you can see what's needed to convert your code.

martin