I did a search on keywords such as "WSG", "Flag", "Hiding" and "Wall Jump" and didn't find this exact hiding spot so I thought I would make my first post here and share it. To some this may be old news but it was new to me when I discovered it and I have never seen anyone hide here before (I have been playing WoW long since before BGs even came out in the first place).
Outside the Horde base, there is a watermill with a wooden wheel that spins. Below this wheel, there is a slanted, stone wall. If you jump up this wall you can actually fall down to the other side of the wall beneath the wheel.
No one can reach you unless they wall jump as you did (I have tested with Warriors attempting to charge, etc). The only exception is that if you get tab-targetted by a Warlock or Hunter, they can send their pet after you and it will path through the wall.
Once you are ready to cap, you can easily wall jump back out by facing the wall you jumped over to get back there and wall jumping up the wall directly to your right.
I'll post pics when I get home from work tonight.