This sounds like a bad idea.

You don't want the employer to waste your time, so you should try not to waste theirs. If you want to work for them you should go in assuming they're not trying to deceive you either.

You counter offers for a dozen other things, why not time? All those other negotiables can just as easily blow the "deal" apart but aren't considered a waste of time/deceit. They often aren't pre-aligned on. Even if you pre-aligned on something, the interview process itself can change your opinion. For example, you may agree that a salary of $x would suffice. During interviews, you determine the job is going to be much more demanding than originally expected so you now say you like it but $y is the new price. It's not deceitful, it's quite normal.

I agree your example of a salary adjustment is fine, no deceit there.

The deceit comes from pretending to be qualified for a fundamental job requirement that you'll never meet (the requirement to work full time).

A better example is applying for a job that requires a license and after weeks of interviews you finally get an offer just to turn around and say "actually I don't have a license and I'm not willing to get one, but I hope your offer still stands?". Except it's even worse becuase at least in that case the employer can verify your license right off and avoid wasting everyone's time.

Lying on a resume or application is just not a good look.

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.