i would pay them in cash or not at all. the times that i have guest bearistaed it was either cash or goodwill. but if you didn't want to pay im sure you could work out some other sort of deal. things such as room and board. beer, you could give them lethal amounts of beer.
but when it comes down to it, to make this really really work, money will probably need to be involved. your average barista doesn't make enough to run off and make coffee just for the hell of it.