Instead of redefining it, can we rename the listing environment and define our codeblock environment in terms of the listings environment? Or is that not possible due to the way that listings monkeys with the input stream?

I don't know what you mean. We already have \lstnewenvironment{codeblock}{\CodeBlockSetup}{}, above.
The problem is that I can't put the "detect marker" code into \CodeBlockSetup, because that's too late and I can't get at the preceding box anymore. In order to get control earlier, I'm redefining the codeblock environment to get an earlier grab.

What I'm asking is whether we can instead use (say) \lstnewenvironment{codeblockimpl}.... and then define a new codeblock environment in terms of codeblockimpl. Modifying an existing environment like this, rather than composing this environment out of the existing one, is a hack that we should avoid if we can. (Maybe we can't; so be it.)

Tried this:

\newenvironment{codeblock}
{%
  \ifhmode%                             no last box in vertical mode
    \setbox\@signalbox=\lastbox%        grab and consume last box
    \copy\@signalbox%                   typeset the last box again
    \ifhbox\@signalbox%
      \ifdim\wd\@signalbox=0pt%         zero-width box is our marker
        \def\codeblockbeginpenalty{100}%
      \fi%
    \fi%
  \fi%
\begin{codeblockimpl}}
{\end{codeblockimpl}}

Result:

! Extra }, or forgotten \endgroup.
\egroup

l.228 ...( ) < > % : ; . ? * + - / ^ & | ~ ! = , @
\textbackslash@ " '

So, the environment nesting appears to negatively impact @ recognition. No idea why.