It doesn't work like the CRJ, doesn't depend on brake position at all.
The main cabin door being closed is recorded by ACARS, regardless of brake position or other doors (service, cargo, etc). It's on the 2nd page of the OOOI screen (Out, Off, On, In).
When the aircraft exceeds a certain speed for a certain amount of time (I'm told it's >4 kts for >3 seconds but nothing in writing), you can see the "OUT" time appear on the ACARS (on the 1st page). This is what they want to use.
Problem is after the main door is closed, the slides have to be armed, cross-checked, flight attendants verify everyone is down, then you run the last of the before start checklist items and request a push clearance.
You're GOING to give up 2-3 minutes at least every single push and, in large cities or with some pax who insist on getting up to get something out of the overhead or a couple bags that show up late, you're going to hit 5 minutes pretty regularly.
Additionally, the flight attendants have a "me, too" clause in their contract. We give this up, and the flight attendants give it up, too, unwillingly and, in all likelihood, pretty unhappily.
It's a lose-lose proposition and does nothing but save the company money.