I did this a few months ago and it's worked incredibly well. I went from full-time to two days per week, leaving me the other three to work on my own stuff. Had a lawyer go over the intellectual property agreements for me first to make sure I wouldn't be in questionable territory.

It all depends on your manager, the mood of the company, and how useful you are. For me the stars lined up perfectly. The CEO is an entrepreneur and wanted to help me out. I was also a critical member of a small engineering team, so they'd rather get two days of my time than none.

One key when you're having the conversation is to frame it more as part-time vs quitting, rather than full-time vs part-time. Having somebody that isn't in the office all the time makes planning more difficult, so it's in your manager's best interests to keep you full time. If that's removed from the options list and it's a choice between having you part-time vs not at all, part-time starts to look a lot more attractive.