Hello,
The best option we can see for that is to use PlantUML to export a diagram to XMI format.
Since XMI is XML, it would allow you to easily parse the result.
Currently, the XMI export is really experimental (see
http://plantuml.sourceforge.net/xmi.html )
However, it's difficult to find documentation about Sequence Diagram and XMI, so if anyone has some input here (or example of XMI files), please post here!