The official answer from the HPI boys is that the interest and enthusiasm is there, but the difficulty and time (and high cost) comes in the licensing of official products. They all know what is wanted by the Trans Am freaks, and the popularity of this class, so I'm sure they are doing their best to produce bodies that their customers want to own.
And I do have it on good information that should a new HPI T/A body become ready for market, we'd hear about it first in the USVTA.