People are probably willing to pay you to do this, but not advertising it so much. Most tech companies (at least in the US) are desperate for work. Less than 40 hours is a week is a tough sell because many people feel the onboarding process eats up a lot of time. Primarily, since it's the norm to work 40+ hours (and believe me, it's not difficult to do this and be actually writing code more than 50% of the time) employers expect it.
> I sincerely think it is quite unrealistic to ask a knowledge worker consistently deliver at full throttle for 40 hours every week (never mind the enslaving 60+ hours on many companies)
I really disagree with this. Have you tried before?