First of all, a clarification. Adding a value in "TeX Local Root" in the Execution Modes interface (TeX Options page) does nothing more than setting a WinEdt internal variable called
TeX-Local which is not used in the defaults.
You can use its value (when set), for example, for adding the local TEXMF directory to WinEdt's input directives, Tree interface and so on.
This variable is accessible inside scripts as
But in your case, I think you are looking for a method to retrieve the contents of the environment variable
TEXMFLOCAL and this is certainly possible.
In fact, an environment variable is accessible in WinEdt scripts as
%@('EnvVar');. So it suffices to use
"%@('TEXMFLOCAL');\scripts" to have the path you need.
Note that in certain cases (when the above variable needs to be expanded) it might not work. In such a case you need to fully expand the path by adding
@@ before it, that is to say
@@"%@('TEXMFLOCAL');\scripts" (this has been introduced, IIRC, in v7.1).