I think in practice this is going to work about 0% of the time.
In the past I would tell recruiters “hey I meet all the criteria and the salary even looks good, but I’m going to need every Friday off”. The salary looks good @ 32 hours/week, but I’d be underpaid at 40. No one has gone for it. They probably end up with someone not as good (because they’re willing to work for below market) who realistically works at most 32 hours/week anyway. But hey, at least the perception that they’re a good hire is in-tact.
It's because they don't know you yet. It's easy for a gatekeeper like a recruiter to shut the door. The hiring manager, if they do actually like you beyond butt in seat, can get creative. Typically they just have a budget and a unit of work to accomplish, they get to fill in the details.
Also, trying to negotiate 25% less work for same price is not what I suggested. It has to be 1:1 relative for them to view it as equitable. They have a lot of things to think about when considering what's "equitable". If their plan was to grind you for 80 hours and pay you for a "full time 40" then they weren't being equitable or forthright either, and it's very common, so it goes both ways.
In the past I would tell recruiters “hey I meet all the criteria and the salary even looks good, but I’m going to need every Friday off”. The salary looks good @ 32 hours/week, but I’d be underpaid at 40. No one has gone for it. They probably end up with someone not as good (because they’re willing to work for below market) who realistically works at most 32 hours/week anyway. But hey, at least the perception that they’re a good hire is in-tact.