I haven't set up a test for this, but if you are able to access the WorkItem IDA2A2 from the transitions tab (or wherever you want to run the validation from) you could query the database for the comments using a Query Spec. I'm assuming that the comments are persisted once the user clicks the "Complete Task" button.
QuerySpec qs = new QuerySpec();
int classIndex = qs.appendClassList(WfVotingEventAudit.class, false);
ClassAttribute ca = new ClassAttribute(WfVotingEventAudit.class, WfVotingEventAudit.USER_COMMENT);
SearchCondition sc = new SearchCondition(WfVotingEventAudit.class, WfVotingEventAudit.WORK_ITEM_REFERENCE + "." + WTAttributeNameIfc.REF_OBJECT_ID, SearchCondition.EQUAL, new Long("12345")); //Change to WorkItem ida2a2
qs.appendSelect(ca, new int[]{classIndex}, false);
qs.appendWhere(sc, new int[]{classIndex});
If the comments are empty, follow with logic to force comments.