Rialto is currently enabled on both stable2 and sprint branches, but an old version of it might be used by default. To build the Rialto's HEAD into the image, one needs to add "rialto_head" to DISTRO_FEATURES of the Yocto build.
Rialto is already integrated into appsservice. To enable rialto connection from the widget to the RialtoServer, one needs to add "rialto" capability in the widget's config.xml.