[ovi256]

Fabian Bellard built an LTE basestation using SDR (so almost everything is done in software). So a GSM mobile stations should be possible, as it has less capabilities.

http://www.bellard.org/lte/

Again, as you said, legalitites. A DIY device would be difficult to approve by whatever telecom supervising agency your country has. That's one of the reasons the current ecosystem of blackbox GSM modems (such as the one used by this Arduino phone) exist. These modems run closed-source software that can theoretically react to network events, possibly get updated OTA. So there's the icky part from a security viewpoint.