Which is the most efficient way to do alpha mask in opencv?

I know OpenCV only supports binary masks.
But I need to do an overlay where I have a grayscale mask that specifies transparency of the overlay.

Eg. if a pixel in the mask is 50% white it should mean a cv::addWeighted operation for that pixel with alpha=beta=0.5, gamma = 0.0.

Now, if there is no opencv library function, what algorithm would you suggest as the most efficient?