This plan is clearly unworkable. Once the gate is open there will be nothing to prevent others using it. Since anybody can press the button and say they have limited accessibility they would have to open the gate and everybody would follow them.
More likely, with lifts across the bridge for accessibility coming in the next couple of years, they will not provide buzzers on the Perry Vale exit. They have previously said it will be manned during peak times. But with trains every 4 minutes I cannot imagine they would man the exit in all weather.
Interestingly from 2010 the station staff will be TfL not Southern, so anything we are told by Southern has limited applicability. But this is something that the Forest Hill Society and local councillors are discussing with the rail authorities. We have a couple of interesting meetings coming up at the end of this month at the station and in Westminster.