540 Defense is ridiculously easy to acquire and maintain, if you put the effort forth. Make (or have made) and keep a set of tempered saronite (410 Defense). Run HoL until you get your trinket (65 Defense). Have a Titansteel Shield wall crafted (64 Defense).
All of those numbers are without enchants/gems, and put you at 639, which is 50 points away from what you need to remain uncrittable. They're also acquirable before setting foot in a single heroic. In addition to, you have 7 slots + gems + enchants to bring you higher.
I would venture a guess that with quest rewards, craftables, BoEs, and non-heroics alone will give you an insane amount of wiggle room to maintain your 540 defense.
Edit: I just went through quest rewards, normal instance drops, BoEs, and craftables, and found another 162 defense for 6/7 slots (second trinket slot is pretty baren for defense gear until you get Thaddius to play nice). The two expensive pieces of gear for the 801 defense rating I found are the gun (that you WILL keep) and the titansteel shield wall (that you probably will use for a while). If that isn't wiggle room to upgrade your gear to stuff with lower defense, I don't know what is.