How can I produce the output of a carriage return in a \message?

You don't say, but \reserved@a suggests that you are using latex in which case

\message{ab^^Jcde} 

works although it is better to use latex constructs such as \typeout or \PackageInfo

In plain TeX the above \message will work if you set

\newlinechar=`\^^J

The \message primitive expands input in the same way as \edef and needs something which produces a 'raw' new line to get the desired effect. On the other hand, \\ is a command to produce a line break in typesetting, so fails here (it is also not expandable, hence the odd error). You therefore need to insert a character equal to \newlinechar. In the LaTeX format this is ^^J:

\message{a^^JB}
\stop

In formats where this is not set (such as plain), you'll also need to cover that

\begingroup
  \newlinechar=`^^J %
  \message{a^^JB}
\endgroup
\end