I think QScreen::grabWindow won't work because it does what it says : grab screen pixels.

Among the several warnings in Qt's doc I see :

The grabWindow() function grabs pixels from the screen, not from the window, i.e. if there is another window partially or entirely over the one you grab, you get pixels from the overlying window, too. The mouse cursor is generally not grabbed.

We want to get a specific window's pixels whether or not this window is at the front (and not covered by another one). We even want to be able to grab invisible figure pixels.

I think a way to get widgets drawn is to have them first rendered onto a suitable pixel based QPaintDevice, using QWidget::render method, capture pixel values and then let our OpenGL renderer draw those pixels before we recapture them with the current method.

Code to reproduce:

figure (1)
demo uitable 1
F = getframe (gcf);
figure (2)
imshow (F.cdata);

It's also not possible to print uiXXX objects, but that's probably a different issue.  I'm not too worried about it because Matlab is removing support for printing uiXXX objects in a future release.  However, they do say that one workaround is getframe() and that isn't possible with Octave.