chrooted apt doesn't know about backported packages

I'm using prevu 1:0.5.0-0ubuntu1~intrepid1. For me bug #63255 appear to be not completely fixed. There is a repository of backported packages and appropriate line in sources.list:
deb file:/var/cache/prevu/intrepid-debs ./
However in order to allow pbuilder to automatically install backported dependencies (libbar as example in bug #63255) an `apt-get update` command is required in chrooted environment, which is not done. Currently I use the following workaround (of course, need to be run only once):

echo ' HOOKDIR="/some/where/pbuilder_hooks" ' >>~/.pbuilderrc
mkdir -p /some/where/pbuilder_hooks
cd /some/where/pbuilder_hooks
( echo '#!/bin/bash'; echo 'exec apt-get update' ) >> F20apt-get-update
chmod +x F20apt-get-update

It would be nice if something similar would be done by prevu automatically.