Control TI Launchpad over USB
http://github.com/dwelch67/msp430_samples I use mspdebug to load programs.  

I dont know if there is a communcation path between the target mcu and the host.  the mcu in the middle is the one you talk to and it uses spi by wire to load the target device.  

I either bitbang or use the uart in the g2553 part to communicate with the host using a separate ftdi based usb to serial board (the kind used with 3.3v arduinos are super easy to come by, can also power the msp430 with this if you dont need to reprogram it.