While it’s a bit of a fiddle, you can do this without a keyboard too. Under Settings->Player->Language in the main menus, set the subtitle position to Fixed. Then, when the video is playing, bring up the On Screen Display, go into Video Settings, scroll down and select Video Calibration. Click OK to go past the first two options without changing anything; then, on the third calibration screen you can set the vertical position of subtitles. Then click Back until you’re back in the video.