Установка ~x86 для группы пакетов?

И никогда не прокатит. Знак '*' может заменять только версию пакета. Это описано в man 5 ebuild:

                     Extended Atom Prefixes [!~] and Postfixes [*]
                            Now to get even fancier, we provide the ability to
                            define  blocking packages and version range match-
                            ing.  Also note that these extended prefixes/post-
                            fixes  may  be  combined  in any way with the atom
                            classes defined above.  Here are some common exam-
                            ples you may find in the portage tree:

                            !app-text/dos2unix
                            =dev-libs/glib-2*
                            !=net-fs/samba-2*
                            ~net-libs/libnet-1.0.2a

                            ! means block packages from being installed at the
                            same time.
                            * means match any version of the package  so  long
                            as  the specified base is matched.  So with a ver-
                            sion of '2*', we can match '2.1', '2.2',  '2.2.1',
                            etc...  and not match version '1.0', '3.0', '4.1',
                            etc...
                            ~ means match any revision  of  the  base  version
                            specified.   So  in  the  above  example, we would
                            match versions '1.0.2a', '1.0.2a-r1', '1.0.2a-r2',
                            etc...

Что касается твоей проблемы, то могу предложить заполнить package.keywords с помощью скрипта. Например, такого:

# cd /usr/portage
# find kde-base/ -maxdepth 1 -exec echo {} ~x86 \; >> /etc/portage/package.keywords