Writing a script to de-interlace only part of a source?

Using avisynth 2.5 and CCE, I'm backing up an episodic PAL DVD that is progressive, except for the credits, which are interlaced. First up I tried what I did in the past with this situation, encoding the stream as interlaced (this doesn't matter with PAL progressive stuff, it's just a flag) and zigzag. However, as I have capped the credits bitrate at 2000 (average bitrate is only 3175) and they have a graphical background they look rather bad. The poor encoding of the original DVD doesn't help either (shame on the BBC). So I'll de-interlace in order to up the quality a little. There's 5 discs with 3 eps on each, so instead of encoding the 2 parts of the video separately and merging, I'd like to write a script that serves up the progressive part and then uses FieldDeinterlace()on the credits part only. Is there some way to do this? i.e. specify filters to be applied only to a certain portion of the source. I did search but it's hard when one doesn't know what words to input. Thanks :)

should do what you want. However, it is pretty buggy in its usage (not taking filters without arguments, yawn, etc), so I'd recommend instead.

Thanks very much for the reply, I'll see if I can figure this out =)