Lily Allen 's getting clever with the marketing for her new album ... using a beloved childhood character to push a song allegedly roasting her estranged husband.

The singer-songwriter stepped out on Halloween night with some pals in Los Angeles ... dressing as Madeline -- the intrepid French heroine from a series of children's books. Getty

Allen looks just like her in her baby blue jacket, bright red wig and straw hat ... and she stopped to pose for the cameras before heading into the Vas J. Morgan Halloween party.

While it may seem like a benign costume on the outside, it's a reference to Lily's new album, "West End Girl" ... and a specific track where she plays out a call with the person David Harbour allegedly cheated on her with -- and, the track's titled "Madeline." G

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