Unfortunately, Gtk is not really suitable for use in plugins on platforms where a "system Gtk" can't be relied on. Some toolkits are suitable for static linking, but personally, I am somewhat disillusioned with "toolkits" lately, and massive libraries in general. Sometimes all you want or need is a straightforward standard graphics API and some keyboard/mouse events.
When it comes to standard cross-platform graphics APIs, the undisputed heavyweight king is OpenGL. While not perfect (what is?), no other API is already there on almost any platform you'd care about (heck, most modern phones have hardware accelerated OpenGL). Unfortunately, OpenGL deals only with rendering, and not user input or windowing issues. What is needed is a minimal framework to get an OpenGL view to draw to, and receive keyboard and mouse events. Enter Pugl.
Pugl handles all the platform specific business behind an API very similar to GLUT, but much smaller and appropriate for plugins (which GLUT unfortunately is not). In terms of size, this is a few hundred lines of C per platform (on a personal note, this fits in well with my ever-increasing distaste in dealing with bloated junk with tons of dependencies... give me a Couple Hundred Lines of C™ any day). The breakdown on Pugl is on its homepage, but suffice to say after a few days' work it does the job it was designed to do on X11, Mac OS X, and Windows. Since embedding X11 works in LV2 land, that means an OpenGL plugin UI can be embedded in the host, and I have the pretty pictures to prove it:
This is a simple test plugin (complete with gratuitous use of 3D) embedded in Ingen and Ardour. Both are Gtk based programs, but this works in Qt as well. Testing so far on other platforms has only been top-level since I have no programs to embed in, but the bulk of the work is done. This includes full keyboard and mouse support, with significantly more complete keyboard support than GLUT.
Naturally I can't predict the future, so it remains to be seen how much OpenGL UI action we'll see for plugins. Being just a low level drawing API and not a set of boxed widgets, it's a bit open ended with a bit of a learning curve, but on the other hand there is lots of existing OpenGL code out there. Perhaps someone will throw together a library of audio appropriate widgets, if one doesn't already exist. Either way, I think an easy to use API for writing truly portable LV2 plugin UIs is a very good thing, which hopefully eliminates a barrier for some plugin developers and helps LV2 invade the territory of its proprietary adversaries... or, at the very least, makes for a really cool 3D panner GUI :)