the single-quote expansion for bash(1) (like $'whatever') supports a \c escape that can be used before an arbitrary byte. its intended to be used for control codes, and masks the lower 0x1F bits of the prefixed byte. so, you can use \c@ as an alternative way to specify NUL (\0) or anything less than 0x20.
posix bash bourne bourneagain

you can find the implementation inside ansicstr(...) here: https://github.com/bminor/bash/blob/master/lib/sh/st...

and the macro for TOCTRL(x) here: https://github.com/bminor/bash/blob/master/include/c...