It seems the suggested way is doable.
It also seems, that using OpenGL ES as the base is a rather platform independent way.
Making work freetype fonts and ftgles to be done.
The complete implementation of themed Tk support is not yet started, it should be relatively easy using enough time to do the work, as the existing implementation for Tk is available.
It seems not to be possible to use native fonts with OpenGL ES.
The implementation of the widgets using OpenGL ES commands is at the beginning.
There is the need for test cases.
There is also the need for examples/demos.