An application note (PDF!) from TI on creating a custom flash-based bootstrap loader (BSL):
MSP430F5xx and MSP430F6xx devices have the ability to locate their bootstrap loader (BSL) in a protected location of flash memory. Although all devices ship with a standard TI BSL, this can be erased,and a custom made BSL can be programmed in its place. This allows for the creation of using custom communication interfaces, startup sequences, and other possibilities. It is the goal of this document to describe the basics of the BSL memory, as well as describe the TI standard BSL software so it may be reused in custom projects.
This application report also includes a small demonstration BSL that can be used on MSP430G2xx devices. An entry sequence starts the code update and allows the new user code to be sent and stored in flash. A one-byte feedback is provided to indicate status. TA0-based UART communication is used for entry sequence, data, and feedback.