I've read the specs for /var/ in the Debian Policy. That's why I came
up with /var/cache, it seems to match my requirements exactly.
Sure, but nothing (besides dpkg) is supposed to modify files packages
ship. (Configuration files under /etc are a special case.) Best
practice would probably be to ship an empty directory under /var/cache,
ship the stock dictionary in /usr/lib/libcifpp or /usr/share/libcifpp as
appropriate (depending on whether it varies by architecture), and have
your postinst script copy the stock dictionary into the /var/cache
directory as needed.