I don't really know (yet?), but it seems to me that what you need is a network of nodes, such that each node automatically downloads a common amount of money from a pool once a month. That pool would be filled by the system subtracting a percentage from the gross income of each node. Is it really necessary that those calculations are done by a central unit - like the IRS -, or could they be done in a distributed manner, SETI@home-style?