In a prior version of the game module, there was an NPC tile type that had the following code, but that still required player interaction. There may be ways to trigger the code with an invisible monster.
Code:
;;; NPC TILE
LDA gamepad
AND #%00000010
BEQ +
LDA textBoxFlag
BNE +
LDA #$01
STA textBoxFlag
+:
Hmm... That might be possible, have the text appear upon invisible monster spawn or collision.
Will have to play around some more. Have only spend a few hours (so far) with NES Maker. Still learning the in's and out's of how everything works. Joe's tutorials have been a great help so far.