Including OpenCV in the distribution is obviously not feasible, since it's too large. But is there a way to use an installation of it from within WinPython?
Yes! I just managed to use opencv-2.4.8.exe from within WinPython-32bit-18.104.22.168.exe.
For this, I followed this description:
I copied opencv\build\python\x86\2.7\cv2.pyd to WinPython-32bit-22.214.171.124\python-2.7.6\Lib\site-packages\, then opened Spyder and tested import cv2.
So this ticket can be closed.