Camping at Byron last Xmas and after the swell had died I went for a paddle from Clarks beach down to the wreck for a look on my 14'. There were plenty of holes and I heard a bloke caught in a rip yelling for help with his 2 kids under 10. I was able to cover the 30 or so metres in no time and get both the kids on the front of my board holding onto the handles and back to the beach. As you say I think they could be a great piece of rescue equipment for patrolling behind the break over a reasonably wide area.
I rescued a nipper that had got into trouble at one of my local beaches. I was on my SUP and could clearly see he was in trouble and could hear him calling for help, but the people on the beach couldn't see him in the surf.
From my point of view SUP rescue boards would be a great addition to SLSA.