Ask HN: Freelancing for equity

Recently one of my freelance clients mentioned they want to offer me some equity in their company. They asked me to make a proposal for this equity deal.

Currently we are negotiating a support contract for a web & app platform I have built for them. I was thinking of reserving x support hours every month for the next year or so that are paid for by the equity deal. Is this something that has been done before? How do I define the value of the equity so I can put the correct amount of services against it?