>How can you demand monetary compensation for working on an entirely voluntarily basis?

It's actually quite easy. There's much ink regarding this issue as this has come up regarding Internships and the like, so is nothing new. As a new user I don't know what I can link to off-site, but there's for instance specific rules in California for what is allowable for unpaid internships and if that's not met, those volunteer interns can file successful unpaid wage claims.