Answer by Kharlos Dominguez for Overcoming the 1024 character limit with setx

This open-source SetEnv command-line tool is good to edit the PATH and other environment variables without limitations. It uses a dynamic buffer so no static limitations like 1024.

http://www.codeproject.com/Articles/12153/SetEnv

The choice of a % as a prefix to append to a variable could have been better though, as makes the syntax difficult sometimes if used with other batch local variables...