Hildon Input Method frameworks is based on Gtk-IM framework if I recall correctly. And it is very tightly coupled with GTK and that's why maemo's native onscreen keyboard works only in GTK apps. And I'm not even sure that it sends key events to the textfield directly because of word suggestion support.

Anyway, it seems that full-blown UI toolkit will need a couple of years of work And I don't see any reasons to consider UI toolkit as a goal.
Lets consider some practical things, like ASUI or Simp, as a goal.
If audioplayer will require text input — we will need to investigate how to implement it. If no — great, lets give up on text fields.

By the way, I have working demo with Seekbar & Player buttons from Kroll's screenshot, with cool seekbar handler transparency

You can grab the source code and track the development at github: https://github.com/IlyaSkriblovsky/lx