Yes it is possible, however you are not guaranteed that OpenGL will render to backbuffer memory for rendering underneath the hidden window on all platforms.
On some platforms there are ways to ensure you draw to the occluded backbuffer, but you could also render to an offscreen buffer rather than the window backbuffer.
You can certainly have one thread render to a progress dialog and another render to the backbuffer, or even schedule the context switches yourself from a single thread, or avoid context switches altogether (probably best and safest IMHO) by keeping the progress bar in some native format.
You could also draw to the backbuffer and render the progress to the frontbuffer occasionally using glFlush, and switch back to the backbuffer… all in a single thread with no context switches but it would require testing and it would still impact performance. It would keep the code simple and avoid the whole occluded buffer rendering problems, but it hides a lot of crap the driver would be doing to make most of these schemes work.