> so mimicking the changed prompt would require querying the subprocess for its sys.ps1 and sys.ps2.

Is that sensibly possible? Any line of code can change those, so IDLE would have to do the equivalent of
idle.ps1,idle.ps2 = sys.ps1, sys.ps2 before every new ps1 statement prompt. A configuration option might work better.
  
I would rather remove the prompts from the entry window itself, and use 4 char indents instead of 8, so that cutting does not pick up the prompts.  IDLE does not have to exactly imitate the Command Window interface. It could also give the user a choice.