As gamerlay maintainer, I'd be glad to introduce some changes to
unpacker.eclass:
1) merging unpacker-nixstaller (Makeself  subspecies) from gamerlay:


# @FUNCTION: unpack_nixstaller
# @USAGE: <files to unpack>
# @DESCRIPTION:
# Unpack nixstaller generated files
# They're shell scripts with the blob package tagged onto
# the end of the archive. In the blob placed tarballs with
# actual content.
#
# Please note, if you need additional dependecies make sure to unpack
subarch
# archive as first argument.
#
nixstaller_unpack() {
        unpack_makeself

        local unpack_files="$@"

        for i in $unpack_files ; do
                unpack_banner "$i"
                # Make sure that file exists
                [[ -f "./$i" ]] && (
                        local type=$(file -b ${i})
                        case ${type} in
                                data)
                                        tar -xJf "./$i"
                                        ;;
                                gzip*)
                                        tar -xzf "./$i"
                                        ;;
                        esac
                ) || die "Failed to unpack $i"
        done
}


Original author is Azamat Hackimov aka winterheart (in CC).

2) It'd be also nice to detect mojo installers (most of the time,
they're "*-bin" or "*Installer" ELF32 sfx-archives) and pass them to
unpack_zip by default.
Unfortunately, I've no free time ATM, but if it is needed, I can write
prototype for Mojo unpacking.