Ok, I managed to compile the kernel module successfully. /usr/share/doc/kernel-package/README.headers file was the starting point to understanding. The file contains a hint about using -I to compile against the exact kernel headers required. I also had to use the -nostdinc flag to gcc, and add other paths to the -I list in the Makefile as I discovered them missing. (Missing as a result of -nostdinc.)
In the end, it wasn't as easy as I had hoped, but at least I managed to work out the problem. The worst part is how long it takes to generate the kernel-{source,headers,docs,binary} packages -- I need a faster machine. Here's hoping someone else is saved the few hours it took me to find the Right Answer to the problem. :) -- Earthlink: The #1 provider of unsolicited bulk email to the Internet.