Provided your router has enough storage (e.g. external USB HDD), it shouldn't be a problem if you're familiar with the command line.
Your router is most probably running either an ARM or a MIPS processor. The appropriate debs can be found in the Debian repositories here. You can't install them directly on the router, so extract with dpkg-deb -x filename.deb destination-dir first.
You can then try moving the extracted files to your router, and seeing if the binaries appear to work. If not, you can always (cross-)compile them from source. I don't see anything in apt-cacher-ng that should prevent it from working on a non-Debian-based system.
Alternative: Use the OpenWRT packages
There's also an apt-cacher-ng branch for OpenWRT on Github here, and it looks like it's being actively updated. DD-WRT runs on a superset of the hardware supported by OpenWRT, so it would almost certainly compile and work on your system. This may be the easier option.
armhfshould work on the BCM4718. Please accept as answer if this helped you out. Thanks! – ish May 27 '12 at 10:27