So for the purposes of this and the extroot-ready image, I think that the following is a reasonable image recipe. Please comment/correct if there are any thoughts/concerns about it:
make image PROFILE=tl-mr3020-v1 PACKAGES="block-mount kmod-usb-storage kmod-usb-core kmod-usb2 kmod-fs-ext4 kmod-scsi-generic kmod-usb-ohci"
This will create an image with the necessary components for the extroot, but that process must still be done manually. I believe that some people have made images that include scripts that auto-extroot to a 'new' USB stick (or more accurately, post-boot insertion of a usb thumb drive initiates a script that formats the drive and performs the extroot process). I have not bothered to create auto-extroot scripts, but I have created some scripts that streamline my own installation process for use as a portable wifi + VPN router.