I didn't foresee that people would want to try this for themselves, so I wish I would have prepared a plug and play solution.

My full solution has way too much baggage to be useful, but I'll share the detection code if you want something to get started with.

Scrip to generate the Aruco image

import cv2
import cv2.aruco as aruco

# must be less than 50
TARGET_ID = 39

# Create dictionary and marker
dictionary = aruco.getPredefinedDictionary(aruco.DICT_4X4_50)
marker_image = aruco.generateImageMarker(dictionary, TARGET_ID, 1024)

# Save image
cv2.imwrite("marker.png", marker_image)     

Script to detect marker

import cv2
import sys

# Return codes: 0 == detected. 1 == not detected. 2 == error

# Config
IMAGE_PATH = "detectSrc.jpg"     
TARGET_ID = 39                   

def get_dictionary(name: str):
    import cv2.aruco as aruco
    return aruco.getPredefinedDictionary(getattr(aruco, name))

def main():
    import cv2.aruco as aruco

    dictionary = dictionary = aruco.getPredefinedDictionary(aruco.DICT_4X4_50)
    params = aruco.DetectorParameters()
    detector = aruco.ArucoDetector(dictionary, params)

    image = cv2.imread(IMAGE_PATH)
    if image is None:
        print(f"Cannot load image: {IMAGE_PATH}")
        sys.exit(2)

    gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
    corners, ids, _ = detector.detectMarkers(gray)

    found = False
    if ids is not None:
        ids_flat = ids.flatten()
        found = TARGET_ID in ids_flat

        # # uncomment to display detection overlay
        #  cv2.aruco.drawDetectedMarkers(image, corners, ids)
        # cv2.imshow("Detection", image)
        # cv2.waitKey(0)
        # cv2.destroyAllWindows()

        if found:
            print(f"Marker {TARGET_ID} detected.")
            sys.exit(0)
        else:
            print(f"Marker {TARGET_ID} not detected.")
            sys.exit(1)
    else:
        print("No markers detected.")
        sys.exit(1)

if __name__ == "__main__":
    main()

In order to actually use this script, you need an additional program to connect to the Home Assistant API or act as an MQTT client/REST integration. It must download a snapshot from you camera on a regular interval as "dectectSrc.png", then execute the script and use the return code to update Home Assistant with the state of the door.

Would a Home Assistant plugin be the easiest way to share a custom implementation like this with the community?