There is software out there to do this, airlines and internet cafe's use systems like this to give short leases for internet access etc and those who are paying a premium service get unlimited access time although filtered at times to only allow access to non black listed sites.
How many people would connect?
Are the same systems always connecting for internet access to where you have a known MAC address list for systems authorized or is this a network that is open to total strangers accessing your network?
What do you have for a budget to buy a solution?
