PKP does NOT sell online tickets , except since last week or so for the trains from Berlin, from out-of-country places and infact for international FROM PL is also still very, very restrictive.
PKP generally also only starts selling tix from 30 days before-due to the enormous nr. of repairs/works/late minute decided timings.
As a very rough general rule CD will have a small quota for its own clients on such cars, again probably only just from 30 days before. You could mail CD, who is very responsive, and ask them to detail/quote the possibilities. That it can sell space to Züri on its own cars says nothing for this.
To me it seems extremely highly unlikely -as so many English seem to fear all the time, that these sleep-cars are sold out on such a date.