According to comments in http://ospublish.constantvzw.org/tools/inkscape-plugins-in-python, it is unfortunately not possible to access Inkscape built-in functions from Python plugins. Hence such functions would need to be implemented again in the Python plugin. This is not so trivial for ungrouping as the group may have transformation applied. I may implement it in Python though, if I find the ungrouping function in Inkscape source code.
According to comments in http:// ospublish. constantvzw. org/tools/ inkscape- plugins- in-python, it is unfortunately not possible to access Inkscape built-in functions from Python plugins. Hence such functions would need to be implemented again in the Python plugin. This is not so trivial for ungrouping as the group may have transformation applied. I may implement it in Python though, if I find the ungrouping function in Inkscape source code.