If this "secret" area 31 is where I think it is, you don't need a ladder. You're also trespassing.
In fact using a ladder anywhere around BFI
on a non-show day is liable to generate a vist from a gentleman in a uniform. If 31 is in use I'd stick to the Museum parking lot. There's a gap by a gate where you can get line-up shots and if you stand on the terrace outside the cafe you're above the fence line. Southerly traffic is more problematic; roll-out shots through (not over) the fence at the Midfield Park by the tower is about the best of bad options. Some departures may be high enough to clear the fence though.
The place at PAE
is the Future of Flight parking lot; sun behind you and mounds to see over the fence. Only good for Southerly traffic, I've never found an acceptable location on the other side.