Voluntary? In an absolutist sense, yes. In practice, no. In theory all work in voluntary, assuming you can leave job and still afford to pay for basic amenities like food, housing, transportation, and still provide for your family, etc until new employment is secured. If you cannot do these things, it ceases to be voluntary.
"but paying homeless people $3 an hour is barbaric, what do you want, homeless sweat shops?"
Well according to this logic, $2.50/hr is better than $0/hr; $2/hr is better than $0/hr; $1/hr is better than $0/hr, and so on. Everything you describe is a race to the bottom in terms of wages. There would have to be a very strong deflationary pressure to bring prices to the point where the above is capable of really purchasing anything. In such a case, the economy would be in shambles anyway, so employment in general would be a huge issue.
What you describe is not new and, at its core, totally exploitative. Living in a company town is strikingly similar to this. The employer actually "pays" the worker, $7/hr, but keeps the $4 majority the payment of ground rent, food, etc. Of course, you leave any time--that is until your meager savings are depleted by a raised cost of living--one manufactured by the benevolent boss/mayor!