That's how Harald Welte and Osmocomm team was always providing on-site GSM networks on events like Chaos Communication Congress/Camp.

However, recently Germany sold out part of spectrum that was used for those licenses, so it became much harder now there. The network on last CCC almost didn't happen, but the provider who bought it (T-Mobile IIRC?) allowed to use it this time anyway. The future is a one big question mark though :(