Probably there is nothing more easy to do than copy hex file to flash card in order to write new firmware to microcontroller. Andrzej found this functionality on some NXP microcontrollers and wanted to make something similar on Stallaris Launchpad. Stellaris Launchpad already has an USB that can be used to set up mass storage.
Bootloader takes around 16kB of flash memory – not so much comparing to total size. After flashing it all you have to do is to press SW2 button while plugging the board and it will be recognized as small external drive. Copy firmware to it and bootloader will load it in to program space. Compiled and ready to use bootloader can be found on Google docs.