Heading down the rabbit hole…
Qwikiverse maps the references between stories: what Batman: The Long Halloween nods to, and what nods back. How it works
Wikipedia: Batman thelonghalloween.jpg
http://comicbookdb.com/graphics/comic_graphics/1/9/7580_20051110200105_large.jpg
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Nothing spotted here yet.
A line quoted, a cameo, a character named after something: if Batman: The Long Halloween nods to another story, or another story nods to it, spot the first one.