For what its worth, the plate covers which are provided by Lakin Plates are in fact approved for use in QLD, although the letter is a bit old.
In addition, the plate covers have a neoprene seal which presses around the raised rim of the number plates. So installed correctly in conjunction with the Lakin brackets, there should not be any possibility of moisture getting in. As for heat... I wouldn't imagine that would be an issue if there is no moisture. Numberplates in direct sunlight get pretty damn warm anyway. Behind a UV treated plastic, they're unlikely to get as hot - although sure, they'd take longer to cool after they've been heated up by direct sunlight.