Where in the shows, novels, doujinshi, etc. is Ryoko ever officially named with the last name?
Come to think of it, where did Washu get it? DK was just recapping the Washu novel and says that she's just referred to by the name but no explanation of where it comes from is given.
Do any of the Doujins list her full name?
I'm pretty sure that I remember her being named in full in an episode of Tenchi Universe (Time and Space Adventures), I think. But it's been a couple of years since I've watched Tenchi. And, taking this with a grain of salt, because we know that Tenchi Universe is a spinoff, rather than official canon.
The only reference I can find in GXP is in GXP 9, Episodes 16,17 (Retranslated), pg 100, when Ryoko tells Seina, "That woman is not my mother!"