By the way, what I have done in the past is have an ongoing agreement of a few hours per month minimum charge (e.g. 5 hours/month) for support, which is not exa

By the way, what I have done in the past is have an ongoing agreement of a few hours per month minimum charge (e.g. 5 hours/month) for support, which is not exactly like a royalty. If they don't need any maintenance, great, you write documentation or do security upgrades or whatnot. If they need more than that number of hours per month, they pay you for it, but they agree that you will get at least that many billable hours. But, this would be something for a future piece of work, it would be hard to do it now.