With the move of X from a prefix of /usr/X11R6 to /usr it has become harder to swap back & forth between a CVS Xorg version (For testing of a problem fix reported to Xorg developers) and retaining the ability to move back & forth easily. previously a symlink, ldconfig & restart was sufficient. now you must de-install, & re-install gentoos X packages to go back & forth. Modular X just makes this more cumbersome as it's not just a single package any more.
Install your CVS stuff in /opt/xorg, prefix /opt/xorg/bin to PATH and add /opt/xorg/lib to LD_LIBRARY_PATH.