Well, the 'unixy' way is to set the permissions in a startup script. Don't have access to a Mac (yet :0), but on Linux/Solaris, there are a set of directories (/etc/rc[123456].d or /etc/init/rc[123456].d). Each script beginning with "S" is executed as root in numerical order when the corresponding kernel run-level is entered. Typically the scripts are symlinks to a script in the init directory (typically /etc/init.d). If similar directories exist on OSX (may or may not, since BSD used to use monolithic files...), create a startup script to set the permissions you want.
Step by step:
1) In /etc/init.d, create the file 'SetBPFPerms'. Here is a skeleton:
-------8< Cut Here 8<--------
#!/sbin/sh
#

case "$1" in
'start')
 #Place commands to set perms here
 ;;

'stop')
 # Place commands to be executed on shutdown here
 ;;

*)
 echo "Usage: $0 { start | stop }"
 exit 1
 ;;
esac
exit 0
-------8< Cut Here 8<--------
2) Edit the file to set the permissions as necessary.
3) Assuming you want the permissions set entering run-level two, create symlinks from the proper directories to this file. On Solaris, the commands (as root) would be
ln -s /etc/init.d/InitBPFPerms /etc/rc2.d/S99InitBPFPerms
ln -s /etc/init.d/InitBPFPerms /etc/rc2.d/K99InitBPFPerms