The only thing I can think of is for third parties.... If you are registered as a third party, you have to make representation to Elections Canada already; you're part of the system. As acknowledged, as a third party you have requirements under that. If you kept foreign funds completely separate from domestic funds—for whatever reason they're coming, whether it's for a service or whatever—when you get to the point of spending for regulated activities, you know that the money is only coming from money that you received domestically.
They're also subject to audit. If the punishment is bad enough, it would discourage them from doing it.