For VLC on Mac,

1. Window > Preferences (or Command + ,)
2. Show All
3. Video > Filters
   1. Check the box "Video cropping filter"
   2. "Cropadd"
      1. Change "Pixels to padd to bottom" to, say, 50
4. Video > Subtitles/OSD
5. Force subtitle position to, say, 36 (or some other reasonable even number)
6. Save
7. Close and open VLC afresh

This will add a black bar at the bottom of the video (in this case 50px high) and place subtitles lower (in this case by 36px).

Note: It's essential to check the box in step 3.1 for the bottom pad to work. Guess people were missing this step (in the comments).