Hello Phil
I am not sure what application you are wanting to modify? The PC application (KinetisUpdater), the bootloader itself (on the KL27) or the application to load with the bootloader.
However, KinetisUpdater is not target specific because the target reports all details needed to it so that it can operate.
If you are developing a bootloader version for your own KL27 you can in fact get ready developed ones from
http://www.utasker.com/kinetis/FRDM-KL27Z.html
which includes KBOOT UART, KBOOT USB, USB-MSD (composite with KBOOT USB), SREC UART, SD card, "HIDloader.exe" USB, AN2295 Freescale Developer's Serial Bootloader loading for almost any Kinetis and IDE.
Regards
Mark
Kinetis: µTasker Kinetis support
KL27: µTasker Kinetis FRDM-KL27Z support / µTasker Kinetis Capuccino-KL27 support
Boot loader: http://www.utasker.com/docs/uTasker/uTaskerSerialLoader.PDF
For the complete "out-of-the-box" Kinetis experience and faster time to market