Flash avb_custom_key using EDL

As documented here , many bootloaders will allow flashing a custom verity key to the avb_custom_key partition using fastboot. This then allows a custom chain of trust so the device will boot a custom ROM. I have a phone which has a locked bootloader so I cannot flash the avb_custom_key using fastboot. But, I do have Qualcomm EDL mode access with a firehose loader. Does anyone know if there is any way to use EDL to flash avb_custom_key? It does not show as a partition in the GPT. Google calls it a virtual partition. I don't know what that means.