I'd say that the best scenes in Brook's adaption to emphasise the nihilistic interpretation are Act 3, Scene 7 and Act 5, Scene 3.
Act 3, Scene 7
-Gloucester's blinding is rendered in black, a technical device that makes it that much more disturbing.. With only the screams heard, the responder is forced to imagine what exactly has happened. (Also a comment on the human condition. No really.)
-The dehumanisation of many characters is particularly apparent in this scene, with the editing of the 'Servant's' dialogue...
I have served you since I was a child//. As such, his killing Cornwall is presented as a betrayal.
-Rather than stabbing the servant following her scream of 'A peasant stand up thus!" (Important in Marxist interpretations), Regan merely rushes over and beats the servant over the head repeatedly, without providing 'justificiation'.
-In the original text, Gloucester is lead off stage by other servants. This is omitted from Brook's production, with the implication being that of his 1962 stage production. (With Gloucester being left to stumble off the stage..)
Act 5, Scene 3
-Battle scenes between the English and French are rendered with a darkened lens, again indicating chaos and confusion..
-Edgar and Edmund's battle is turned from a 'duel' into a single brutish blow, any traces of 'elegance' eliminated..
-As opposed to other productions, Lear experiences no 'redemption' whatsoever, as seen in his costuming..
-The reconciliation between Edgar and Edmund is virtually eliminated, the only line remaining being The wheel is come full circle. [Eliminating positive elements, a major element in nihilistic interpretations..)
-A small alteration dramatically alters the effect of Edgar's last speech.. It's said before Lear passes, which to me is a really depressing thought, being ignored before passing, alone..
-The final shot is of Lear dying, alone (Contrast to say the Elliot interpretation), with the shot lingering on the empty sky, echoing the empty and bleeding earth of Jan Kott's interpretation..