Getting back to mags' original question. There is really nothing lost if rain falls on your bike overnight, provided you cover your seat (free plastic shopping bag is my choice). In fact if you have been riding on wet roads during the day, accumulating road crud, your bike might get a good clean overnight. Just remember that your chain lube will be whittled away in the wet and you will have to replace it periodically.
Laying the locked bike on the ground beside the tent has never been a problem to me over 6 big rides. After your tent and the surrounding ones have been erected, choose the location for your bike that offers the greatest protection from walkers. Even better if your neighbour places his/her bike in the same gap. That way it becomes too hard for walkers to navigate and they choose another path with less resistance.
The only time I had a problem in bike stowage is when a bloke persuaded me to turn our bikes upside down and chain them together. A strong wind came up that afternoon and blew them over. result: chain grease on the tent and a gouged decal on my frame. Never again!
On the matter of which bike to bring. I have the same Giant carbon road bike as mags, now 4 months old. It will be staying safely chained to the spare bed at home. I will bring the 4 year old aluminum trek road bike. I like these big rides but I am not prepared to bring the best bike. In fact the reason that I bought a new road bike was to release the previous best for these tours. The Great Vic ride last November was my first with a road bike. I will never take a mountain bike again, the road bike is just so much lighter and more responsive. In short, more fun.
By all means bring your Giant if you want to mags, you will see more expensive bikes than that on the ride and it is most unlikely anyone will report any regrets. I am just a little cautious in these matters.