procedure PatchRoutine(ASystemRoutine, AOverrideRoutine: TFarProc);
const
  JUMP_FAR = $E9;
var
  Protect: DWORD;
  OldProtect: DWORD;
  Address: Integer;
begin
  VirtualProtect(ASystemRoutine, 256, PAGE_EXECUTE_READWRITE, @OldProtect);
  if PByte(ASystemRoutine)^ <> JUMP_FAR then
  begin
    PByte(ASystemRoutine)^ := JUMP_FAR;
    Address := Integer(AOverrideRoutine) - Integer(ASystemRoutine) - 5;
    PInteger(DWORD(ASystemRoutine) + 1)^ := Address;
  end;
  VirtualProtect(ASystemRoutine, 256, OldProtect, @Protect);
  FlushInstructionCache(GetCurrentProcess, ASystemRoutine, 256)
end;

procedure MyShowMessage(const Msg: string);
begin
  MessageBox(Application.Handle, PChar('Calling MyShowMessage'), MyShowMessage, MB_ICONWARNING)
end;

//////////////////////////

PatchRoutine(@Vcl.Dialogs.ShowMessage, @MyShowMessage);