Not sure why we don't see more of this, but it is totally possible.
Here are the rules:
1) NPCs spawn the elite guards, they don't just randomly spawn.
2) There are a couple of pats you have to watch out for (ie. Donnelly Firecask) who spawn guards if they see you.
3) The NPCs are tightly packed around the entrances, so you need a rogue (or maybe a feral) to get in. Hunter and moonkin stealth is not strong enough.
4) The ethereals in the transmog / reforge rooms and the bankers do not spawn guards.
Basically, this means entire areas in the shrines can be ganked in. In the Shrine of Two Moons the portal room, bank, transmog room and the inner walkways are kill zones. It is harder in Shrine of the Seven Stars as the pats have further routes and there are NPCs in the bank and portal room which you have to stay clear of, nevertheless still entirely possible.
You can see this in action here: