Hi Alex, both the jBPM Web Designer and the Signavio editor use the core Oryx technology. The Signavio version is AFAIK not open-source and is completely maintained by their developers whereas the jBPM Web Designer is open-source and is based off an Oryx fork from Intalio.
Each runtime implementation, that being for example Activiti, or jBPM is going to have some hooks for extension elements and attributes. The BPMN2 specification fully allows for that. Custom workitems is something that has been in jBPM for a long time. The implementation requires you to specify the TaskName attribute so that it can associate it with the configured workitem handler impls at runtime. The jBPM tooling support, that being in Eclipse or web-based makes creations of workitems easier, something that the Signavio editor may not do...but all of them should produce valid BPMN2.
Also, I do not know what parser the Signavio editor uses for creating BPMN2 processes, nor if it follows the spec or not, so I cannot say more on that. The jBPM Web Designer uses eclipse.bpmn2 API which is also open-source to marshall/unmarshall BPMN2. We have frequent releases and everyone interested is more than welcome to contribute to the project (let me know if you are ).
Hope this helps.
I understand. Using the provided tools to create the BPMN2.0 processes is a must to make sure that the extensions are really there then.
The Signavio editor based on Oryx is actually open source. The project can be found here: http://code.google.com/p/signavio-core-components/.