Soooooo, I have heard back from McLaren, the odd thing is that STW (& I) appear to be wrong
Dear Geoff,
This is a very interesting question and one we have never been asked before, although people have talked for years about the possibility of F1 cars running upside down in a tunnel, which is possible, so going vertically up a wall took a bit of thinking about!
Unfortunately, it is not possible and the reason is that the engine is not quite powerful enough to drive the car quick enough to generate the down force that is necessary to produce the traction that is needed at the rear tyres, given that the engine has also got to overcome gravity on top of everything else, but it could go up at slope at 85 degrees.
I hope this answers your question,
Kind regards
McLaren Racing Limited