Lily Allen Appears to Takes Another Shot at David Harbour, Dresses Up As Madeline

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.

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."

Take a listen for yourself ... Lily's basically relaying a call with "Madeline" where she asks her if her relationship with Harbour had emotion in it or if it was just sex. Lily claims she and David had an open relationship -- with conditions that he must pay for sex and he had to keep it low-key.

Ultimately, Allen sings that Madeline isn't a stranger like she should be ... which is why this relationship crosses the line.

Lily's making a big marketing push with the new album ... including handing out butt plugs at an event this week -- referencing a line in her hit song "P***y Palace" from the same album. The plugs are allegedly USBs that contain the album.