Soda blasting wont do anything to a rubber coating. It's only effective on paint.
Al, I'd just take them to someone that does media blasting, and have them remove the rubber coating. There's a guy near me that uses dozens of different media, and he is fantastic. He even removes paint, varnish, etc, from antique furniture without damaging the grain. He knows a lot of different tricks. For example, he told me that he recycles his media, because different media develops different characteristics once it has been used, and those new characteristics can then be used for other unique applications. Small pieces, like your scoops, are usually inexpensive to do. He did a complete Challenger fender for me, inside and out, removed all of the rust and bondo, and sprayed it with a protective coating that keeps it from rusting, all for $100. I'd say try something like this first, before you go experimenting on your own. I know you are a do-it-yourself kind of guy, but, like you said, the pieces are rare and expensive, so I wouldn't take chances just to save a couple of bucks.
