I don't have much for you regarding equity. I'd propose thinking about other ways for you both to find a winning relationship:

Is your customer actually making money? One thing you might propose is some kind of profit share based on your contribution. I.e., propose that you take a % of future profits. This would be a great incentive for you to do a great job for them; and the premise is that everyone is making money. By this means, you might avoid some of the agony of trading your own time for something unpredictable.