Malcolm Dew-Jones wrote:

> I wish to type Ctrl-@ while using an xterm (perhaps it's a clone) and get
> the NUL char sent to the application, as in most other terminal-like
> interfaces.

Does it work with ?
(ctrl-v being configured in your TTY with "stty lnext")

Forget about the ctrl-modifier in X11. It is hardcoded
in this concern and unrelated to the TTY control usage,
e.g., ctrl-d==ASCII-4. You don't need a KeySym-name;
just map it literally, example (mapped on shift-space):

xmodmap -e "keycode = space 0x0"

> I think that `at' refers to XK_at in keysymdef.h.

Yes. (see for the literals)