[org.clojure/clojure "1.7.0"] [quil "2.3.0"]
This is a non-ideal Quil sketch within a custom JFrame.
Essentially a lot of the older messier PApplet was broken out into various other classes. If you search for .getNative() you'll see a whole lot of places where it now lives. The .getNative appears to be available with a running sketch, however I could not simply create (.getNative (PApplet.)) as there seems to be quite a bit more other stuff going on now elsewhere is those other classes.
So this example blinks the main sketch frame momentarily, which is unfortunate.
But it proves that it can be done. I suspect that tracing the sketch initialization, and possibly recreating a lot of what happens up until the point of when the main graphics object is added to the frame may be key.
This will probably require a Java based wrapper class, and that may be easiest in order to cut & paste as much of the existing code as possible in order to make it easier perhaps.