Found the solution myself eventually
In MCUXpresso, under Project -> Properties -> C/C++ Build -> MCU settings, in the memory map, I had to change the location of my flash memory from the default 0x30000000 to 0x60002000
The micro boots now!
Various other details in case anyone else runs into this in the future:
* I added an XIP_BOOT_HEADER_DCD_ENABLE=0 line, which previously did not exist (it wasn't set to 1, it just wasn't there at all)
* The file type that finally worked was an .s19 generated as a post-build step
* I changed the Size of my flash memory from 0x10000000 to 0x0ffe000
* I was able to use the MCU BootUtility's Boot Device Memory tab to write and read back binary files well before the micro was actually booting