The symlinks that are placed in /usr/lib/opengl/mtx/lib/ by the mtxdrivers-pro ebuild contain absolute pathnames. The update-opengl utility, however, requires the links to be relative as it prefixes with an absolute path that it constucts itself. Ergo, the opengl-update utility cannot be used to swap to the mtx implementation without doing some manual editing first. This is broken behavior.
Should be fixed in 1.1.0_beta-r1.