You could consider changing your frame rate and then restretch the file to the desired frame rate. Let's say that currently you have one second of animation, but you want it to take 1/2 second.  Render the movie at 15 frames per second and then in a video editor, retime the clip to double-speed on a 30 frame per second timeline.  

It's a kludge work-around, but it should do what you're looking for.

 - Dex