Subtitle (soft) positioning support for MP4 (mov_text)

I am trying to understand the limitations of soft subtitles and MP4 containers. I want to add a mov_text encoded subtitle stream to an existing MP4 file and I am hoping to control the color, style and mainly the position of the subtitles.

I currently test playback using VLC media player.

Is positioning supported by mov_text encoding? I couldn’t find much information on the spec. If so, is this possible to do using GStreamer?