It may also be helpful to review the statute on lobbying, and see what it says about these office-holders. I can't remember offhand, but they may delineate office-holders for whom reports must be provided by people who have access to them. That may be helpful as well.
I was also thinking of executive assistants, and I'm not thinking only of ministers. I'm thinking of leaders of parties as well. It may be that the chief finance critic of a party is also someone whose staff has importance. It may seem as though that goes a long way, but then it's much easier if you make it such that all these things have to be reported anyway, in an exceedingly timely fashion, within a day or two and shared automatically from one web to another. Then the Chief Electoral Officer wouldn't have to go look at the Conservative Party website to find out if an event was held; he or she would automatically be given that information and could act accordingly—immediately—to ensure that people were following the law.