Mini-HowTo, wie man backports.org-Pakete per APT-Proxy installieren kann:

How to install backports.org packages with APT-Proxy

APT-Proxy konfigurieren in / Configure APT-Proxy in
/etc/apt-proxy/apt-proxy.conf:
add_backend /backports.org/ \ $APT_PROXY_CACHE/backports.org/ \ http://www.backports.org/debian/

Beispiel-Zeile für / Sample line for
/etc/apt/sources.list:
deb http://192.168.1.1:9999/backports.org sarge-backports main

Pinning in
/etc/apt/preferences:
Package: * Pin: release a=sarge-backports Pin-Priority: 200

Und nun einfach installieren / To install just type
apt-get -t sarge-backports PACKAGE

Big Thanks to Norbert Tretkowski for his work!