Mac OS X "hosts" file, can I include other files with it?

I am not aware of any include possibility. What I'd do, however, is to make sections in my hosts file, and then use a script to comment the lines in each section using, e.g., sed.

This way your file would look like

#%%%HOME.HOSTS%%%
#Put here the contents of home.hosts
#%%%WORK.HOSTS%%%
#Put here the contents of work.hosts

#%%%ALWAYS_ON%%%
127.0.0.1 localhost
::1 localhost

Edit: Adding in a quick attempt to modify the fields.

Removing the comment for HOME.HOSTS

 sed -i '/#%%%HOME.HOSTS/,/#%%%/s/^#\([^%]\)/\1/g' hosts

Putting back the comments for HOME.HOSTS

 sed -i '/#%%%HOME.HOSTS/,/#%%%/s/^\([^#]\)/#\1/g' hosts

This is a basic version, and needs to be adjusted to your needs.