My suggestion is to add somewhere in the configuration dialog when users
can enter a block of Python code to be executed at startup and whenever
Restart Shell is executed.

Use case: for people who use IDLE for calculations/experiments they
might like to always have certain module imported. For me personally, it
would be:

   import os
   import re
   import sys
   from math import *

but of course the whole point is that people can write any code they
like. (Some people might want to do various from __future__ imports in
Python 2.6 to get various Python 3 features for example.)

I know that you can use the -c option, but that only works at startup,
not every time you Restart Shell.