Is it possible/convenient to reply to the conference PCs regarding the meta-review result of a paper?
I assume that the committee has also seen the reviews and hasn't objected to them. At least a subset of the committee has likely looked. Presumably they agree with the decision. 

You can communicate with them and ask for a new review, of course, but I think you are not likely to get it. They will most likely want to follow the established procedures which, as you say, don't anticipate a rebuttal. Conferences also have time constraints and the submission/review schedule feeds in to other things that also have time constraints. They are also pretty likely to trust their reviewers. My experience with annual conferences is that the set of reviewers is fairly stable and committees also overlap from year to year. 

On the other hand, I also doubt that they would withdraw the acceptance as a poster as long as you don't come across as abusive or insulting.