Hey!
I wrote a patch for the "adduser" utility on FreeBSD 5.1-RELEASE. I sent a PR to the FreeBSD team to try to get this officially added to the release; you can see the PR here (http://www.freebsd.org/cgi/query-pr.cgi?pr=60636).
Anyway, the patch lets you create home directories with permissions other than 0755 by default. Go to http://snow.cs.siue.edu/~unix/adduser/ and get both adduser.sh.diff and adduser.8.diff.
You are free to get the shell binary and man page I have there, but I don't know if they will be doing updates to adduser any time soon (probably not).
Once you have these somewhere, do "patch /usr/sbin/adduser adduser.sh.diff && gunzip /usr/share/man/man8/adduser.8.gz && patch /usr/share/man/man8/adduser.8 adduser.8 && gzip /usr/share/man/man8/adduser.8". Now you will have an updated adduser script and man page for it.
Let me know how this works for you!