Hi Dreadnought!
(You're hopefully not called Andy, too?) ::)
You're clearly well ahead of me with regards to the stage you're at during the build of this ship, and I've not given a vast amount of attention to the details of the superstructure as yet - so the following may be taken with a pinch of salt.
What I can tell from the online photos of the Dreadnought is that jpg artifacts just about rule out seeing anything of the flag halyards, but there's the odd hint that (like the Colossus) the halyards did not need to be vertical, and that they ought end up somewhere sensible: that would suggest an area very near the flag lockers (and you'll know these were moved over time). Probably the best bet would be to find an original photo of the ship that you can use a magnifying glass on - or check on the as-built plans in the NMM.
There are certainly a number of holes in the AotS book - notably "what's happening" between turrets X and Y. I know the basic facts for this area, but the building at the bottom of the main mast, the chequered decking and intake hatches just aft of X are not as well described as they could have been. That's nit-picking, though...the book is fantastic, otherwise.
As to the roof of the Admiral's cabin, my build is the post-1915 refit, so I've got to build the larger sea-cabin which was presumably metal. The earlier wooden structure always strikes me as
a shed stuck on at the last minute. Probably somewhere for the Admiral to make his model boats.

That said, I'd go for the corticene option - especially if the roof has stanchions around it, or it could be used as a surface for going upwards.
One more point on the AotS book - the deck planking shown for the tops goes in a different direction to that portrayed in one of the photos. I don't know if this is a post-refit thing, but it's a point that I think Deans may have got wrong.
Do you have any photos to inspire us slow-plodders along?
Regards,
Andy