Hello Mike,
-There isn't USB DFU demo under LPC55sxx. There is usb_device_dfu demo under MCUXpresso SDK for LPC54608,
you can have a look.
-And if you just want to update image, you can also make your chip enter into ISP mode, then can through USB , UART or other interface to program, about detail please refer to UM:

-There is a second bootloader , you can have a look, it for lpc55s69.
https://www.nxp.com/docs/en/application-note/AN12327.pdf
Have a great day,
TIC
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