WikiProjects associated with pages, along with the class and importance, should be available for querying from Lua. Perhaps as a pageassessments propery on mw.title objects.
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Please note that the Scribunto patch allowing other extensions to define properties on mw.title objects is currently under review at https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Scribunto/+/1091767.