I've answered my own question. BPMN2 validation can be disabled by not using the BPMN2 Project Nature. This project nature causes automatic validation of all BPMN2 files; many of these rules are 'over-eager' when it comes to validation of Ad-Hoc processes.
When saving edits to a diagram, do not add the BPMN2 Project Nature, causing automatic validation of all BPMN2 files.
Alternatively, disable this using: Properties > BPMN2 > Uncheck "Check if project is configured for BPMN2 Project Nature".
Steve.