It seems defocus doesn\'t work well with splitting... I assume this is the same issue with vector blur and similar \'cubic\' postprocessing kernel effects that use pixels outside the rendered area.
To make use of these postprocessing technologies you will probably have to:
1) Render the scene on BURP without these enabled
2) Render the scene locally with the output from BURP as colour input to your processing system, the z-buffer from the scene as z-buffer input (and the same with vector information). This should be pretty fast as you can disable raytracing, osa etc. for this step.

If you feel like experimenting, please try this out and write up a small HOWTO on the topic.

I tried this for fun today and figured out that everything needed to do this is available in both Blender and BURP. However, the CATS downloader does not yet have the functionallity to correctly name the files in the format expected by Blender for image sequences, so (for now) you will have to manually rename the files when downloaded.
The extra local rendertime added by doing local postprocessing rather than doing postprocessing on the renderfarm is around 5 secs per frame for simple scenes - which is very acceptable.

I added the point about filenames to the CATS \'known problems\' page.