How do I account for my time spent on product development? This time is overhead and not billable. Can I convert it into owner's equity at my normal billing rate?

If this is a sole proprietor or a single member LLC, the equity is the profit, so need to post the cost of your time. Because there is no posted expense, the profit and thus the equity will be higher. If this is a corporation then you would pay a salary to yourself as an employee, (and pay payroll tax on that), and because there is a posted expense, the profit and thus the equity will be lower, as you actually received a payment for your time. If this is a partnership, then your time needs to be accounted for as this affects your share of the profit