Originally Posted by
Banzai
As long as you don't go low-normal (rapid rise) or Dyna-sys, all Shimano rear mechs are cross compatible.
I personally use an XT "shadow" mech with my 105 (5700) shifters for exactly the purpose you are trying.
A low-normal derailleur is also fully compatible, work as well as high normal once you get use to the way they shift.