If you want 10 seconds with 25 FPS, you still get 300 png's!

Hiya!
While it is just a user hack, it may help to solve your problem at the pesent moment. When you want 10 seconds with 25 fps and have a non animated world, just set first the fps, second the length - this way you will get 250 frames and 250 pngs when rendered.
Also, you can do a notepadhack, by saving the animation as .chaos, and then opening it in the notepad, finding the animation length and fps, and editing them in the notepad to the desired values.