I would be neat if you had a way that they could opt to download and run a miner in a pool (Eligius would be a perfect fit since it can payout to any address without registration) and mine enough to pay the access charge... 
I like the idea; Imagine people with smartphones on the streets bitcoinmining for hours to get 1 euro for internet acces, they won't have power left to internet. (would be fun to see tho)