Actually, Beetlejuice was not planned that far in advance. They originally were going to mention it in "Competitive Ecology" (3x03) and Megan Ganz (writer of "Cooperative Calligraphy" aka the bottle episode) mentioned to Dan Harmon that they need to be careful because they had brought up Beetlejuice twice now. Then Dan Harmon went back and realized it was actually the third time, so they moved it to the Halloween episode so that they could make that joke.
It's still the writers being awesome at continuity and background jokes, though.
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"One toot on this whistle will take you to a far away land."
-Toad, SMB3