While running FM Radio you can read the RDS data stream from the sysfs interface of the driver (/sys/bus/i2c/driver/radio_bcm2048/3-022/rds_data or similar, tried to reconstruct the path out of my head). It should be pretty easy to extract TMC there. 3rd party apps can make use of the FM radio driver through the fmrx-enabler D-Bus interface.