The current method used for exporting SubStation Alpha format uses a combination of white space characters and line breaks to position caption text. However, both SSA and ASS are capable of far more precise positioning for text, both through Margin overrides and the "\posX,Y" manual placement tag.
The current WebVTT export includes more precise positioning information using the "line" tag, but player support for placement tags is spotty. Comparatively, SubStation Alpha is supported by far more software and devices. Using similar logic to what is present in the WebVTT export code it should be possible to add precise positioning information to SSA/ASS exported subtitles.