一个简单的例子：

type
  mbf = function(hWnd: HWND; lpText, lpCaption: PChar; uType: UINT): Integer;
  stdcall;

procedure TForm1.Button1Click(Sender: TObject);
var
  h: integer;
  mb: pointer;
  ms: TMemoryStream;
begin
  ms := TMemoryStream.Create;
  ms.LoadFromFile('f:\user32.dll');
  h := memLoadLibrary(ms.Memory);

  if h > 0 then
  begin
    mb := memGetProcAddress(h, 'MessageBoxA');
    if mb <> nil then
      mbf(mb)(handle, 'test', 'test', mb_ok);
  end;

  memFreeLibrary(h);
  ms.Free;
end;

代码未经测试，只是转载备用。