A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 5 March 2009
I couldn't find any answer to the question at hand so here it is.
Is it legal to seppuku a samurai/courtier who was targeted by Bayushi Saya's ability after assigning to battle?
Seppuku: Any number of times per turn, before you lose Honor from an ACTION that targeted or was performed by a Courtier or Samurai personalities: He commits seppuku. If the seppuku destroyed him, reduce the Honor loss to one.
Since assigning to battle is not an ACTION neither PERFORMED by a personality, he can not commit seppuku.
Brook Cunningham (rules official), 5 March 2009
Is it legal to seppuku a samurai/courtier who was targeted by Bayushi Saya's ability after assigning to battle?
Yes, it is legal.
Your proposed answer to your own question is incorrect; you are losing honor from the delayed effect of Saya's ACTION (which targeted your Personality, fulfilling the Seppuku trigger), not from the assigning itself.