In any case, external UI's should be avoided if possible, there are very
few valid use-cases. Just use a 'normal' LV2 UI or let the host generate
one.
I'm sorry, but I have to ask - what is for you a "normal" LV2 UI?
Some might say Gtk2 or Qt4 or X11 on Linux, but things are changing
fast that will not allow this.
Gtk3 UIs will not work on Gtk2 hosts, with the same regarding Qt4
and Qt5 (and vice-versa).
Then don't forget we're moving to Wayland and/or Mir...
Current Qt5 release (as of 5.2) doesn't provide a way to embed
widgets afaik, like the Qt4's QX11Embed/Container.
In my case (of Carla 2.0), I'm moving away from having *any* toolkit
code in the backend except "native" UIs (Linux being X11 for now).
This means UIs that use some toolkit (Gtk2/3, Qt4/5, FLTK, etc) and
need instance-access will not work.