I discovered that the mystery bootloader (see previous post) is from a Mediatek (urgh, again?!) MT6260. Oddly, flashable through USB1.1 instead of serial like I was expecting.
The download agent detected the external flash as a 32Mbit Winbond, and read it without trouble. I get the feeling there's a bit missing as I assume there to be internal flash in the SoC... but I don't know yet.