I've used the crop tool in RawTheraee to crop a raw image:
Is there a way to full-screen preview the cropped section prior to exporting?
Doesn't look like. The closest I got to achieving the effect you are looking for was to hit m to hide all side panels, F11 to go full screen, and then play with the scrolling wheel until the crop area filled the screen. Not perfect, but kinda works.
EDIT: As an alternative, darktable will behave differently with regards to the cropping. Once you selected your crop area, it will take the whole screen for you to work on.