how to capture telnet traffic in “cooked” (per-line) mode?

How can I capture traffic just in "cooked" mode?

"Cooked" mode is called linemode, so you'd need to use a Telnet client that supports linemode, talking to a server that supports linemode, with the client configured, if necessary, to use linemode, and capture while those clients are communicating.

z/OS's Telnet server apparently supports linemode and advertises it when you connect to it. The OS X telnetd man page implies that it can be compiled with linemode support; it's probably a fairly standard BSD Telnet server, so that probably applies to many other UN*Xes as well, but I don't know whether that's the way it's compiled on OS X or any other UN*Xes. I don't know what other servers do.

The OS X telnet man page says

 Once a connection has been opened, telnet will attempt to enable the
 TELNET LINEMODE option.  If this fails, then telnet will revert to one of
 two input modes: either ``character at a time'' or ``old line by line''
 depending on what the remote system supports.

and it's probably just a fairly standard BSD Telnet client, so recent Telnet clients on other UN*Xes probably also support it. I don't know what other clients do.