@lps I have not tested this out, but via some reverse engineering tricks I know I think this should get you pretty close. Just create a new FOG iPXE menu and add this in as the parameter.
chain -ar ${boot-url}/service/ipxe/boot.php?mac0=${net0/mac}&arch=${arch}&platform=${platform}&sessname=bob || goto MENUYou will need to replace bob with your session name for the multicast image push. That should join you right into the session with one pick on the fog menu.