It seems like the only difference between phantom and real stock is the paperwork. I think one can have a few shareholders while keeping things simple, but more and there's more reporting required. So if the legal documents are good, go for it, if you can afford to do the work without receiving cash. The benefits are that you can say you have a customer and get their feedback and testimonials, plus payment later if they're successful.
Yes, it's just like extending credit, which is all many startups have. And, you may look at their long range plans, get them to the point where they qualify for funding, and put into the agreement that you get a certain amount of cash at that point.