I understand that the SDK uses that method to preserve proprietary features, but if one does not need those features, does the VOXL 2 bootloader allow for loading custom images into the "boot" partition? The device shouldn't care how the image is built, and so I mainly ask about the feasibility of flashing unsigned images (regardless of how they were built) to the "boot", "rootfs", and "user" partitions with something like fastboot.

VOXL 2 uses the "Qualcomm method" for builds and we rely on many of Qualcomm's proprietary features which does not lend itself well to full open source.