Well you'll need an air compressor no matter what, it doesn't provide energy it just converts it for you. In one case you would have an expanding gas driving an engine that would drive the compressor, in the other you would directly power the compressor via a pulley system from the bike, you could also use the bike to generate electricity and use an elecrtical power to power the compressor, this would be advantagous in that you could use a transformer as a gear box instead of a more complex direct connection, but it would be higher loss.
Chemical reactions tend to slow down when the pressures increase so direct cehmical powering is probably not a good idea, steam is a good idea, lots of power to be had there, however you need to heat it with a combustible, which isn't very green.
Personally I like the idea of a well thought out and constructed gear system powering an air compressor directly from a bicycle. The single most important part is going to be the compressor itself, and the pressure containers it uses. As I said in my previous post nothing says 'green' more to me than using human power to derive kinetic or electrical energy. Most people need the excercise anyways so it's a health/longevity benefit to the person as well.