I was thinking along the same lines as I've had a bit of experience with Microchip PIC programming but was hesitant to comment.
Normally the hex (bin) file is a sequence of bytes that starts at the origin 0x0000 to the end (depending on the size of the flash memory). 0xffff
Nuvi Regions seem to be containers where the start and end adresses are known only by the firmware.
Does the ARM Processor in the Nuvi have internal Flash like the Pics or does it rely purely on external flash like the early 85's ?