The TradeMe ad for the sale of Big Nick may actually contain a couple of revealing things.
Will Gandalf ride to the rescue of the dwarves (and our titular Hobbit) at Erebor? Seems so.
The first hint is the (hitherto unseen?) photo at right of McKellen (or his double), riding Big Nick across what must be the Ruapehu Desert. The location is certainly not Te Anau. The second is the quote below:
Draysill (Aka Big Nick) features extensively as Gandalf’s horse in action scenes on films 2 & 3.
Ridden by stunt riders on locations across NZ (in film 3 is seen galloping across the Ruapehu Desert and Te Anau locations) and he quietly stood with Sir Ian McKellen on his back in the film studio.
Additionally, Old told TV3 that:
Quite a lot, [McKellen would] be coming from the side and shaking his big stick .. or … staff, and yelling.
So, this tiniest of spoilers? Gandalf will ride through the Desolation of Smaug to the aid of Thorin & Co. And he seems to be doing so alone — no White Council in tow. And he might even fight from horseback during the Battle of the Five Armies.
Time will tell.