Never a formal ruling, but I have.
State of Texas: used CDBG-DR to attend APA National in New Orleans, although it had a really solid nexus to disaster recovery so there wasn't much question.
Prior entitlement city: My salary/benefits was funded 50% from CDBG. I routinely attended state APA conferences.
I have seen individuals from City of San Antonio at the Texas APA Conference that I know are 100% funded through CDBG/HOME, which typically means their travel, training & dues are also funded from CDBG/HOME.