You could use Xerxes-C for C++ http://xerces.apache.org/xerces-c/. 

For Python, you could use http://docs.python.org/library/xml.dom.minidom.html or http://lxml.de/

SVG data is stored as elements in an XML document, so you don't need inkex to read, update, or even create the SVG/XML file. 

XML tools will let you parse the file in, search for the tags & values you want, and update them.

- Susan