>>40887 (OP)
There's been some stuff sort of along the lines of what you're saying in the past.
There was that short film called "Redemption" from 2007 in which a young girl is abused for being a bedwetter and then when she grows up she ends up wearing diapers and sucking her thumb as a coping mechanism. The film was absolute garbage though. Really poor production and the narrator is god-awful.
There was also a horror movie in the 70s just called "The Baby" about an adult man who was raised as a baby his whole life by his demented family.
The picture is from a 1956 movie called Baby Doll (written by Tennessee Williams no less) where the main character sucks her thumb and sleeps in a crib. No diapers, and she's not an ABDL, the idea is just that she's so spoiled and childlike the film just uses these things as symbols to reinforce that.
I've never seen it, but I always wanted to, a film from 1968 called "A Thousand Pleasures": http://www.imdb.com/title/tt0063690/
Apparently it's a pretty surreal film where a guy murders his wife and then goes on some kinky sexual misadventures. There's a lesbian couple in the movie who keep an attractive blonde women in diapers and a crib in their house and call her "baby."
There's also ABDL references scattered around a lot of different media over the years, which you've probably seen referenced on here before.
Personally, I think it's totally possible that someone could make a mainstream ABDL film eventually. There's been openly fetishistic mainstream movies before, with 50 Shades of Gray being the most well known example. Although if you've ever heard of The Secretary it was a critically acclaimed indie film with Maggie Gyllenhall that portrayed a BDSM relationship in a serious and postivie light.
I actually had an idea for an ABDL indie film script that I wrote an outline of years ago. It's definitely possible. I think if someone was going to do it it would have to be done in the same vein as "The Secretary." Focus on the relationship aspects of it. Cast some sexy people. Don't ignore the fact that it's weird but help people understand it. Since it's ABDL it would probably have to focus on the DDLG aspect of things to appeal to normies more, but I definitely think something like this could work.