Hi Jackson7781
The on board boot rom code is never open to external.
If you want to develop your own secondary bootloader, we don't have exact demo for LPC4074, but you can refer it as well.
SPI secondary boot loader
https://www.nxp.com/docs/en/application-note/AN11257.zip
I2C secondary boot loader
https://www.nxp.com/docs/en/application-note/AN11258.zip
LPC1700 secondary USB bootloader
https://www.nxp.com/docs/en/application-note/AN10866.zip
These ANs are based on LPC17xx but motherd is the same for LPC40xx
This AN is Migrating from the LPC177x/8x to LPC407x/8x, in case you need it.
https://www.nxp.com/docs/en/application-note/AN11335.zip
Thanks,
Jun Zhang