Snazy,
Although there are plans for the opening up of the Perry Vale car park as a new exit I am not aware that Network Rail, TfL, or Southern have agreed to implement such a plan.
Lewisham council and the Forest Hill Society continue to raise this with the railway companies at every available opportunity as it would be relatively simple and cheap to accomplish. There area few obstacles to this being an entrance to the station. Lewisham council are sorting out the steepness of the slope to the car park and making it more pedestrian friendly. The two remaining issues I have heard about are possibly blocking the driver's view of the platform and a sub-station close to that part of the car park. I am sure that both of these issues can be fixed, but as yet I am not aware of a definite plan to open this as an exit.