Hey @BeeKeepers ,
Sounds like what you are doing is correct, you should just be able to replace the names as you have done and the game will pick up the changes. I have attached a sample package that illustrates this: unzip and install this mod and Sims will be in the t-pose in CAS. All I did was put an "x" in front of all the clip names in the CAS state machine which makes the point to non-existent clips and so the game unable to find them and we fall back to the t pose.
Hope the example helps. My suspicion is that somehow your modifications are not getting picked up by the game.
-SGMS
P.S. Note that on the next patch, all statemachines will be converted to binary XML, similar to how tuning was converted to binary XML. The game will still be able to read plain XML but the data in the shipped package files will be in binary.