I chose to alter mine slightly, cutting from 37.5 to 35 (I could have gone properly part-time to 20 if I didn't need the money), and everything is just pro rated.
Perhaps it's a Europe vs US thing, but it's fairly common to request changes to your career design to allow for flexible working. Obviously it works best for knowledge worker jobs, but it's also applicable to all kinds of shift-pattern work too.
The argument of course is that if you need multiple people to cover the role, the overhead cost is more. However, on a twofold point: many employers know about the BS around productive hours and don't always need cover (I work with people who work shorter hours than I and they aren't covered), and larger organisations (the unsexy ones) can abosrb this without barely a glance.