Actually, the answer is no.
There is no OBLIGATION upon a T/PC to include planning as an agenda item at a general meeting nor to have a separate committee in order to formulate and submit comments on a planning application. But if you do have such an agenda item or specific committee then it is very difficult to see a valid justification for it to be anything other than a public meeting.
There is no reason why the individual Cllrs should not comment as individual Cllrs direct to the LA planning page - they would be commenting as individual Cllrs not the council.
If this is an unexpected cancellation of a pre-planned planning committee a notice could be posted informing the community that the council committee will not sit on this occasion and any person with a personal comment should submit it them self direct to the LA.
Unless it is a significant / controversial planning scenario, really, "so what" if a T/PC misses some routine planning consultee comments... It'll probably go completely unnoticed...