Okay, well, here's what I would do.
I would just replace the upper left transparent tile with another transparent tile and then go into the tileset editor and assign a terrain tag to that new transparent tile (for example, 1). Then in the event you use to dismount, make a conditional branch which only executes that event if the current terrain tag is not 1 (this can be done with variables. Go to Control Variable: Character [Player]: Terrain Tag). If the current terrain tag is not 1, then just play the buzzer sound effect.
Also, turn stop animation on when you are on the dragon. It looks silly otherwise.