It is not possible to set the "--config" option, because of a missing case part in /usr/share/genkernel/gen_cmdline.sh like this: --config=*) print_info 2 "CONFIG: `parse_opt "$*"`" ;;
Apparently, it's been a very long time since somebody tried to use the --config option, because I'm pretty sure that's been missing there for a long time. I've added it in git. Thanks.
Due to the date of the last comment on this bug I assume that feature to be included in 3.4.10.907 or an earlier release of genkernel. Closing, please re-open as needed.