as far as I know there is no ‘rule of style for this as of yet.

Personally, I would use either parentheses (text) or brackets [text] or more likely I would describe what the person was signing, rather than using dialogue.

example:

“Quick, Lucy,” I said breathlessly, hoping I was enunciating well enough for her to read my lips, “I need you to help me bury a body.”

Lucy gave me a dubious look, and with a couple of slow, sharp gestures signed back indicating that I had better better be fucking joking.