I have reviewed the patch, and apart from setting the promt, I think it would be possible to set it at the beginning with the same approach:

    def getprompt(self):
        self.interp.runcommand(textwrap.dedent("""\
        try:
            print(sys.ps1, end="")
        except:
            import sys
            sys.ps1, sys.ps2 = ">>> ", "... "
            print(sys.ps1, end="")"""))

What I don't know is why ps1 and ps2 are the only attributes that are missing in the sys module within the ModifiedInterpreter. When I try InteractiveInterpreter in the original interpreter, it shows the 79 attributes that sys has originally.