Looks like the folks at Lab599 are producing a paddle that fits their TX-500 radio, similar to the keys available from Elecraft for their KX2/KX3 radios.
While not available yet, the details are available in a PDF file here (link).
Here is their mockup image from the PDF:
Curious about everyone’s thoughts here. I personally am very interested. It would remove the need to carry (and not lose) the small adapter cable that comes with the radio, but I worry that they would need to get the feel and performance just right.