Hi! I'm glad you liked the maze generator.
What I would do is create an array/list with all of the DEAD_END and then try to figure out which one is the farthest away. Not an ideal solution, but maybe the easiest one would be to check the distance in a straight line from the entrance, and choose the one with the greater distance, I think you would get pretty close to the farthest one and it is quite simple to check for that.
Sorry I can't help you more, it's been a while since I programmed in GameMaker, good luck with your project!