Does a year of entrepreneurship in an unrelated field affect an MS application?
I think you are forgetting that you do not necessarily need to self-finance your graduate study.  A teaching assistantship would take care of your tuition and would give you a stipend to live on (frugally).  Teaching assistantships are not hard to come by in the field of CS -- as compared to some other fields, because universities generally need a fleet of TAs to grade and/or teach the undergraduate classes.

You may want to consider putting in some applications for grad school this coming December - January.  You can always quit your current job after collecting a strong recommendation from your current supervisor.  Then you would have February - August to try out your entrepreneurial idea and see how it goes.  If it's taking off and you're really enjoying it, you can always cancel (or possibly postpone) the grad school plans.