Honestly, only you will know how your boss will react.
Are there other employees working part time? Maybe you can approach the subject from a different angle.
When you do approach it, make sure you approach them with their interests in mind. Offer to work full time hours every other sprint or so. Just remember, don't just ask part time, part time in most states is anything under 35 hours. Also, as part time, you're likely to loose any benefits (i.e. health insurance, 401k, etc.).
I think, before you approach the idea of part time at work, you should approach the idea of cutting a little bit of leisure our of your week. What are you doing all weekend? Why not take one day completely off, and work for a few hours after work each day, and devote one of your off days from your full time job to maybe a 10 hour day on the startup?
What type of life have you created for yourself that requires x amount of dollars/pounds/euros/whatever? Is there anyway to cut household costs? Maybe cut out cable and only have internet + netflix? Maybe cut out a home phone if you have one?
Or maybe, rather than moving to part time, ask to tellecommute one or two days a week? That will cut out whatever commute time you have, and save you some gas money, which you can in turn use to cut out your expenses and how much you require each month.
But, honestly in my opinion, if you're wanting your startup to succeed, I don't think you should go into it requiring a certain amount to live on because, from the sounds of it, you're used to living on a software dev's salary (what? ~$60k/year?). There will come a time where you will need to devote full time hours to your startup, but you will NOT be bringing in that much money to take home that salary.
I think you should reevaluate what's happening at home before you reevaluate what's going on at work. You're just as likely to be let go (no matter how important to a project you are/think you are) when asking to get part time as you are to actually get it. It's a risk that honestly doesn't sound like it will work with you.