Kelly
A USB-MSD loader occupies less that 10k of code space and is probably more practical in many cases than the KBOOT-HID mode (needs special PC SW and is fairly slow).
With 18k it is possible to have an Ethernet web server based boot loader which is probably the fastest and often most practical method.
The uTasker serial loader (including the various optional methods) also includes an OS in its size which makes it very modular and easy to maintain. FSL_Kinetis_Bootloader_1_0_2 is built up with call tables and is rather difficult to follow so I wouldn't like to extend and have to maintain functionality. It took 2 days to add KBOOT USB-HID and UART modes to the uTasker serial loader, test on various KE, KL and K devices using several IDEs (noting that the operation is completely simulated in the uTasker Kinetis simulator so no HW target debugging was necessary) but it seems the FSL_Kinetis_Bootloader_1_0_2 method needs to have different projects developed for different devices and there are several months between releases with little new functionality (the PC SW side is still quite Beta in operation and newer embedded code seems to have lost some device support on the way - possibly due to the high maintenance overhead for multiple devices).
There are CAN, I2C and SPI interface modes that look to be planned for the future but I don't really know whether they will be used much since this then needs a HW module connected to the PC for it to be able to work and I would think that anyone developing, for example, a CAN based node would probably rather add network based updates as a part of the standard application than use the KBOOT mode; at least I have never heard anyone asking about the progress of these planned modes - but I continue monitoring to keep up to date.
Regards
Mark