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.
I control Iuchi Takaki and declare an attack. Iuchi Takaki does not assign to attack. Indeed, no personalities are assigned to attack.
I resolve a battle. I can still use Iuchi Takaki's Engage ability from home?
I can target cards not at the current battlefield with the Engage ability?
Paul Ashman
Engage (and also Interrupts) abilities are subject to the Rules of Presence and Location. You may not announce Engage abilities if you don't control units in the current army, unless the abilities have the Absent keyword. You may not use Engage abilities on cards in units not at the current battlefield, or target cards in units not at the current battlefield, unless the ability has the Home keyword or allows to target cards "at any location".
Edit: Also, before the follow-up question... The exceptions to these rules don't cover Engage and Interrupts. This is intentional.