With a few exceptions, most of the BG is on established paths and these are likely to have been cairned by walkers over the years. There are also one or two paths that are infrequently used by walkers but are recorded in guides such as Wainwright's and are also marked by small cairns - the route from Langdale Combe to Rossett Pike is one such, it's also used in part by the Langdale Horseshoe race.

At a rough guess there's the following:
large parts of the Skiddaw - Great Calva - Mungrisedale Common section;
the direct line up Clough Head;
the direct line from Grizedale Tarn up Fairfield;
the descent to Dunmail from Seat Sandal;
the climb up Steel Fell;
the rakes on Bowfell
The ascent of Yewbarrow

I've seen cairns on Clough Head and the Rakes on Bowfell, I've seen reflective wands on the section up Great Calva. There could be more these days.

Personally I think the route is now well enough known that extra cairns aren't needed. I think that those that mark the starts of non-walker sections should be removed - there's enough erosion on the Bowfell rakes as it is, without encouraging more traffic than the BG itself generates. The contender and/or their navigators should know the desired line well enough that they aren't reliant on locating a small pile of stones that may or may not be where they think it is.

As for BG himself having more of an adventure and doing it for fun. Well he did set out with the intention of extending Eustace Thomas' Lakeland 24hr record that had stood for 12 years.