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 August 2014
Can Ninja Stronghold be used to ignore Goju Saido's gold cost, specifically when including his Invest trait?
Ninja Stronghold Gold Cost 4 Keywords Ninja Text When paying to Equip or Recruit a non-variable-cost Ninja, or for a non-variable-cost action that puts any Ninja cards into play, you may bow this Holding to ignore the action's Gold cost.
Goju Saido Force/Chi 3 2 HR/GC/PH - 5 0 Keywords Assassin • Ninja • Shadowlands Text Invest : Melee 5 Attack (this cannot be Interrupted) that may target a card in a player's home (if otherwise legal). This Invest cost cannot be reduced. Lose 2 Honor. After Saido enters play, lose 1 Honor.
Ben Higgins (rules official), 6 August 2014
Can Ninja Stronghold be used to ignore Goju Saido's gold cost, specifically when including his Invest trait?
The prescence of the Invest trait on Saido means he is a variable costs Ninja card (5G or 15G).
This means that Ninja Stronghold cannot be used to ignore his Gold Cost.