You dont need to source stuff. All you need is to manipulate the enviroment variable with setenv command and the run rehash on the next line. If you want this to work proper you want to change the path before maya runs anything.
So define MAYA_OVERRIDE_UI in your system environment variables to some mels script such as myStudioOverrides.mel then in said file put:
yourFunctionToUpdatePaths();
rehash;
// source is a stament if you want it to run AFTER your
// stuff you must eval it. Otherwise it runs first no
// matter where in the file its placed
eval("source initialLayout.mel");
This SHOULD work, didnt test this right now. Seem to recall having done it tough. Remeber you need a different mechanism for batch modes!