Just tested Signavio web editor, it works on IE9 without any issue, your editor is based on Signavio, why yours does not work on IE9, do you have any plan to make it work on IE?
What is exactly that IE does not support to cause the editor does not work on IE?
The Activiti Modeler is based on the open-source version of Signavio. If Signavio decided to port it to the open source codebase, then it will be available.