I ended up figuring it out myself by cobbling together pieces from various sources and experimentation.
My .uuu script looks something like this:
uuu_version 1.4.127
# Load bootloader image into RAM over SDP
SDPS: boot -f imx-boot-<MACHINE>-sd.bin-flash_evk_emmc_fastboot
SDPS: done
# Setup uboot environment for flashing eMMC [must have emmc_dev set in (default) environment, for Verdin IMX8MP, this is set to 2]
FB: ucmd setenv fastboot_dev mmc
FB: ucmd setenv mmcdev ${emmc_dev}
FB: ucmd mmc dev ${emmc_dev}
# Use FastBoot to send the bootloader image over SDP, and make FastBoot flash it into the 'bootloader' HW partition of ${fastboot_dev}
FB: flash bootloader imx-boot-<MACHINE>-sd.bin-flash_evk_emmc_fastboot
# Determine if BOOT_ACKs are desired (appears that BOOT_ACK should be used for eMMC fast boot) [I have emmc_ack set to 1 in (default) environment]
FB: ucmd if env exists emmc_ack; then ; else setenv emmc_ack 0; fi;
# Flash .wic image to main area of eMMC [.wic is a sparse, whole disk image containing the partition table and partition data, generated by bitbake based on layout in a .wks file]
FB: flash -raw2sparse all my-factory-image-emmc-<MACHINE>.wic
# Done flashing eMMC
FB: done
as mentioned in the commented lines above, you have to set emmc_dev (2 for Verdin IMX8MP) and emmc_ack (turned on, ie. 1) in your default environment. I’ve added it to CONFIG_EXTRA_ENV_SETTINGS in my board config header in include/configs/