I said "pay me 3/5ths of the salary you just offered me and I'll work 3 days a week". I also offered to keep this arrangement flexible so that I could work in stretches of varying lengths if needed.
This did not go over well. People were actually offended by this proposition and it went nowhere.
As a founder myself, I understand the desire to only hire people that will give you their full heart and soul. But, I'm also a practical person and if someone's skills are valuable to me, then they are very likely still valuable in part.
In fact, when it comes to a product/engineering position, I think the employer would come out on top in such a scenario. They still get access to the person's brain and all of their knowledge and ideas. For someone that's being offered high compensation, their knowledge and ideas are probably a big part of the reason why.