How to get product attribute value from within Helper\Image or plugin
I'm trying to customize Magento 2.0.2 CE to have Helper\Image getUrl() to return a custom url pointing to an external resource instead of using the Magento file system. I added a text attribute to the product that holds the image URL, but I can't find a way to access the value with either of the customization methods I've tried.
The first method was to override Magento\Catalog\Helper\Image and the getUrl() method. I can obtain the product from getProduct(), and I can find my attribute's object in the array returned from $product->getAttributes(), but I can't figure out how to get the actual value of the attribute from the Magento\Catalog\Model\ResourceModel\Eav\Atttribute\Interceptor object.
The second method was to create a plugin for Magento\Catalog\Helper\Image, but I can't figure out how to access the attribute value or even the product, since getProduct() is a protected method.