I don't know the technical details but it seems to get the info from mvc streams on the fly. There is one more thing from the latest beta version. I contacted the developer of the player to adjust the relative pixels to 3d depth for text subtitles and it seems to be implemented.
ps. Actually, I made a feature request to use OFS file with text subtitles for re-encoded 3d videos as it looks possible and rather easy compared to get the info from mvc streams. However, failed to make him understand.
If the OFS info were merged in mkv container, it would be more practical and accessible information.