The rule would be the same, but that's the rule. If they wanted to mutiny, they could - but they wouldn't have a leg to stand on. Again, it's not an ideal solution, but the cast is being told up front about how we are making the rehearsal call from day one, so it's not going to be a surprise - and at least I am looking out for their interest.
I too have noticed a trend in actor's wanting to tighten their day, but always, when a show is involved, it's always different. (For example, in my five years here - my actors have only voted once to allow me to reduce the break to 1.5 hours before a performance, although they are fine in reducing it to one hour during rehearsal.)
But in reality, since we are planning a seven hour day, we are doing your Lady Bracknell example, we are going to rehearse from 10:30a - 5:30p, but not everyone is called the whole time. I would never see the point of calling people from 10:30a - 3:30p, and then just take a four hour break (unless we rehearsing a rep and a change over needed to happen - which is where I think this rule actually has it's history . . . )
Actually, some actors like to have a more time between rehearsal and performance - go home, walk the dogs, take a nap, have dinner, and then go to the theatre. (note, our rehearsal and performance venues are NOT in the same location.)
If you were to use this without a good reason, then you are probably going to face some sort of mutiny. But, there are lots of ways you can take the rules and abuse your cast (like the way to rehearse 24-hours a day . . . like 7:00a costume fittings, calling someone in for a 10 min shoe fitting during the day and then nothing else.) And the end of the day, a lot of this job is style and how you enforce the rules.