One have normally only access to the last X11 selection.
With xzz you can read/write any number of selections.
Write to your X11 startup script:
xzz  xzz : >/dev/null&
xzz + # append X selection to history xzz ++ # write STDIN to X selection and append to history xzz - # show history in xterm xzz -- # write history to STDOUT xzz  # erase history xzz :& # loop-append X selection to history