Nothing official. And if it had any other USB modes it wouldn't look like a plain SD card anymore. But you could use a knock protocol to talk to it.

Another solution would be to use an EyeFi card. A while back someone hacked a different firmware onto them; at least conceptually you could do that again and have them report no data. Some details: https://www.os3.nl/_media/2013-2014/courses/ot/connor_stavro...

Using an EyeFi to immediately transfer the data to a cell phone with encrypted storage is also a consumer level solution that would just work. But of course phones come under heavy scrutiny too and they might just beat you up until you tell them the pass code.