Hello Tijs,
I think an extension will be the cleanest approach because it will not restrict the “alphabet” of the other fields.
If is hard or you prefer to avoid to extend the BPMN, some prefix or suffix to the process ID or process name could do the job but then the process definition designers should be aware of the special meaning of some symbols in the corresponding field. In addition, the process ID is used often and it would be better to keep it shorter.
Probably the process definition category is also an option, as you suggested in the thread for the definition development version - Support for process definition development version
It seems that the targetNamespace attribute of the definitions field is not used currently, but this also be an abuse and is probably not supported by the BPMN editor. 
Cheers,
Monique