Precomped video layers can't frame/pixel blended. This is a problem and the solution is to copy the timeremap keyframes to the TimeWarp or Twixtor plugins. The problem is that you can't copy them alacarte— AE's timeremap keyframes don't paste properly into the plugins. One has to manual create each keyframe by copying the value from timeramp to the plugin.
A simple script that does this would be great. Anyone?