As presented, this idea seems out of scope for Idle.  First, Idle is concerned with editing code, submitting it for execution, and displaying stdout and seterr output from the execution.  Idle normally knows nothing about name bindings created by user code in the user process. The code for menu commands is executed in the Idle process.  The code you displayed would have to be executed in the user process as part of user code.

Second, your request is for a very specific personal need. Running 
for i in range(1000): print('a'*100)
in 3.4.3 does not make Shell sluggish for me.  How much output, on what system, with what version, is a problem for you?

However, selecting a 1000 line block -- which is at least as likely to be output (as above) as an output representation -- is not pleasant.  A new Select Block option, active in Shell, would be generally useful.  The selected block could then be copied.  Deletion is not currently possible, but I might like to change that too. 


The debugger is an exception to Idle's normal ignorance of user code global and local namespaces.  The debugger window (which needs upgrading) can display names and object representations of both namespaces.  I do not know what it currently does with long representations (say 1000 chars or more), but they  are clearly problematical and might best be truncated if not already.  Once the debugger has already pulled a string representation into the Idle process. reusing the existing Copy (to clipboard) function should be fairly easy.