Of course, most of the time it would be assur. That’s the main halachah as stated in the מקור החיים. The point I’m making is that there could be an exception. As you said, you would need to know (לשער is the word used) that there won’t be a counter argument.
Now to apply it here, what LU said is not ridiculous, that a moderated forum might be an exception (in other words a case in which one could determine – be משער – that there won’t be a counter argument).
So for example, let’s say someone asked for help about something in the CR, and a poster (let’s call him Stringy) replied with advice and chizuk. I do not think there would be a problem with another poster chiming in that Stringy is a very smart and kind person.
I maintain my objection to lists of posters based on your original point that being left out hurts.