I'm sure the processors would process that product for the harvester and take on that section of the supply chain. However, the processor is going to charge the harvester for the processing, or the harvester is simply going to get a wholesale price for those prawns, the same as they get for the rest of their export market prawns. What is lost there is the harvester's ability to achieve a higher profit margin from the direct sale.
As far as the actual preservation and freezing of prawns goes—if we're going to discuss tubs and whether or not there's another route—some folks have frozen prawn tails in zip-lock bags. However, the shell is very sharp and can pierce the bag, and then it leaks and you have freezer burn. There are very few other methods, if any. Certainly there are none that can compare with using tubs. There really is no other viable or reasonable way to freeze the prawn tails.