I go along in formation flight with 411A here - if the cabin staff cannot get ready and set, I sign the report as such, and they will look for a new job...
Our purser always come visit us, and ask for "early seat belt sign ON" if they see that preparing the cabin will take longer than normal. I personally make a PA to the cabin, advise the passengers that they better visit the toilets since I will turn the seat belt sign in the next few minutes... We normally put the seat belt sign ON at top of descend, that gives them 35-40 minutes to landing.
...Or is this one of these airlines where crew communications are stopped by the cockpit door...? Don't ask me why I love cargo airplanes, little boxes never complain, about late arrivals or cabin too warm.