Not possible in AE. Doing so would require to know the stream profiling information based on the compression algorithm's matrices and algorithms which in turn would require to be able to scan all existing frames which in turn would mean AE had to render them and apply the compression afterwards. Neither commodity is provided by AE and even the exporter modules for MPEG and FLV temporarily store as many frames as they need to create a GOP/ keyframe interval... As you see, a classical chicken-hen problem.