Hi -
What I do is copy the following files (which I figured out by running ldd a lot):
/bin/sh
(unfortunately sendmail needs this)
/dev/null
/dev/urandom
/etc/hosts
/etc/ld.so.cache
/etc/nsswitch.conf
/etc/resolv.conf
(to be able to use DNS from inside PHP)
/lib/ld-linux.so.2
/lib/libnsl.so.1
/lib/libnss_compat.so.2
/lib/libnss_dns.so.2
/lib/libnss_files.so.2
/lib/tls/i686/cmov/libc.so.6
Then I got mini_sendmail from here: http://acme.com/software/mini_sendmail/ which is an almost statically compiled sendmail alternative. It doesn't compile correctly under ubuntu, and I'd have to look up what the exact problem is, but if i remember correctly the problem was that a user was defined incorrectly and was pretty easy to fix - ask if you have problems.
You can then move the binary to /usr/sbin/sendmail and you should be done.
Finally, I also have the locale files copied for the languages I need to provide translations:
/usr/lib/locale/...
For updates, I periodically compare the files in one chroot to those in the absolute paths and check for new ones, which of course is far from optimal, but then again they seldom change.
Good luck!