Might it be possible to override ID_SERIAL in a custom rule prior to 60-persistent-storage, thus avoiding duplicate entries while leaving the system ruleset intact? Something like (untested):
SUBSYSTEM!="block", GOTO="wwn_serial_end"
KERNEL!="sd*", GOTO="wwn_serial_end"
ENV{ID_SERIAL_SHORT}!="00000000", GOTO="wwn_serial_end"
IMPORT{program}="scsi_id -xgd $devnode"
ENV{ID_SERIAL}=ENV{ID_WWN}
LABEL="wwn_serial_end"
(The above would do the edit on both disks as well as partition.)