One possibility based on your original idea would be to do this:
Code:
FrameEvaluate("arethesecredits=(current_frame<10000)")
...
Trim(2654,128609)
...
<lots of scripting here>
...
ScriptClip("arethesecredits ? Sharp(1.0) : Blur (0.25)", after_frame=true)
-Vit-'s solution is neat, but would have to be amended if any of the processing changes the clip width or does any kind of frame decimation.