A CAS bust is a different form which the ATCO's use for breach of the ANO and its very rare a pilot ever see's that form and most don't know about it.
I think they just allow the voluntary stuff just so they don't have a seprate system for it.
Personally I would use chirp. MOR's are pretty useless to pilots.
A CAS bust would be reported by ATC using the ATC Occurence Report, which is used for all things MOR related by ATC.
Only if it was felt justified would the additional form CA939 Report on alleged infringements of air navigation legislation be submitted.