Works:

eselect bashcomp enable --global ri

Silently fails:

eselect bashcomp enable ri --global

Reason: (/usr/share/eselect/modules/bashcomp.eselect)

do_enable() {
    #...snip...
    if [[ ${1} == "--global" ]] ; then
        bcdir="${ROOT}/etc/bash_completion.d"
        shift
    fi

    #...snip...
    # ignore any unrecognized options
    [[ ${bc} == --* ]] && continue

Always looks for --global as the first parameter, apparently. Bug? Not sure. This would be the second eselect-related bug I've noticed in the past two days, if so.