This list would very likely be much much larger, had not many previous Tenchi message boards and websites become broken & blanked from existence. If you know of any other topics or missed pieces still available elsewhere, please feel free to contribute.
Tenchi Ban 5.12 - afterword (http://setosfan.yuku.com/topic/491/512-Postscript)
21st Century - House Kamiki Secrets Revealed (repost copy) (http://setosfan.yuku.com/topic/398/House-Kamiki-Secrets-Revealed-repost-after-ezDisaster)
Heisei Ban 3 & 21st Century - Dee & the Precursors (http://theayekafanclub.yuku.com/topic/2766/Dee-amp-the-Precursors)
OVA3 4koma (http://setosfan.yuku.com/topic/450/Onihime-has-4koma)
TTN Yosho - afterword (http://theayekafanclub.yuku.com/topic/1353)
Masaki - sister of the First Emperor of Jurai (http://setosfan.yuku.com/topic/503/Repost-Masaki)
GXP Ban & GXP Ban 1 - character profiles (http://www.deskadoo.com/GXP/)
Kamidake Onsen 5 & CDROM - Seto & the veiled widow (http://setosfan.yuku.com/topic/394/Alleged-Assassinator-Seto-The-Case-Of-The-Mystery-Woman)
Minaho & Tennyo (http://theayekafanclub.yuku.com/topic/453/Kajishima-Onsen-708-doujin)