truth be told I bet a Texture Environment Macro would be more successful then the material macros considering the ease of use a Texture Environment Macro would provide.
The only problem I would foresee which is a problem now, how/who would manage the live db once it becomes overly flooded. Being the nature of what an hdri is ("an image") how would you categorize it in a way that would not result in a huge drop down list. I think if there is ever a plan for a gallery of sorts for the Live db this idea would be the next likely request to add to Octane.
Well just some food for thought.
