The best solution I can think of is to try to use the principles to find that middle ground. For example, you could say that a minister should never have anything to do with a constituent who is coming before a regulatory or other body. But these are politicians, and the nature of representing a riding and a constituency is that you want to help.
One solution I know that has been tried with some success is one in which the minister can write about his or her personal experience with, let's say, someone seeking a licence, in the CRTC context—not to say “I think they should get the licence”, but “if I have had a positive experience, why shouldn't I be able to share it?”—in a way that then is up to the regulator and that is respectful of the integrity and impartiality of a regulator to make a decision.
When I've been asked whether a minister should write a letter as part of a process, rather than say “never”, which I think would be unduly constraining, I would say that there are contexts in which you can do so and respect the integrity of the process and not suggest that you think there's an outcome that the minister is advocating for. It's the advocacy that is the no-go zone, not the being part of a process or sharing relevant information and so forth.
I think if you take that principled approach, you'll find far more middle ground that in fact will satisfy the legitimate interests of ministers as politicians while reinforcing the integrity and impartiality of these important regulatory or quasi-judicial settings. A no-go zone is important, but it's important to limit it to the areas in which this would actually do damage.