@lynne yours aren't actually dead though, you could've read the TRM or asked the linux-rockchip IRC on Libera at any time and we'd have explained to you how it works. So that you're not just feeding shelter cats to coyotes. :^)
That "firmware" is just u-boot+tf-a, all open, can reside on either byte 0 of spi flash, sector 64 of eMMC or sector 64 of SD in that order, and if no magic num is found there it enters maskrom mode and you can give it a loader over usb and flash with it.