I've just been using GSmartControl which is included with PartedMagic and couldn't help wondering if something like this exists for FreeBSD...

Can't help wondering if a FreeBSD clone of PartedMagic could be put together without too much effort. It does seem to be an program which has aggregated lots of third party apps under one GUI but it looks nice and has a lot of useful features for support purposes.
GSmartControl is just a stupid GUI for S.M.A.R.T. Please refer to Wikipedia for an explanation. sysutils/smartmontools is very easy to configure. You need to edit /usr/local/etc/smartd.conf at your liking. For example this is a key snippet of smartd.conf from one of my file servers.
# First (primary) ATA/IDE hard disk.  Monitor all attributes, enable
# automatic online data collection, automatic Attribute autosave, and
# start a short self-test every day between 2-3am, and a long self test
# Saturdays between 3-4am.
#/dev/hda0 -a -o on -S on -s (S/../.././02|L/../../6/03)
/dev/da0 -a -o on -S on -s (S/../.././00|L/../../6/01) -m root
/dev/da1 -a -o on -S on -s (S/../.././01|L/../../6/02) -m root
/dev/da2 -a -o on -S on -s (S/../.././02|L/../../6/03) -m root
/dev/da3 -a -o on -S on -s (S/../.././03|L/../../6/04) -m root
/dev/da4 -a -o on -S on -s (S/../.././04|L/../../6/05) -m root
/dev/da5 -a -o on -S on -s (S/../.././05|L/../../6/06) -m root
/dev/da6 -a -o on -S on -s (S/../.././06|L/../../6/07) -m root
/dev/da7 -a -o on -S on -s (S/../.././07|L/../../6/08) -m root

Once you have smartd.conf at your liking start the daemon.

echo 'smartd_enable="YES"' > /etc/rc.conf
service smartd start

Make sure you configure log section of S.M.A.R.T. daemon correctly so that report get sent to appropriate log file which should be e-mailed to you every day. The good policy is to get those e-mails only when there is a problem.
Caveat S.M.A.R.T. is not monitoring vibrations of the old plated HDD. Vibrations rather than the temperature is the main indicator of HDD failure. Secondly I am not aware of the any product which uses automated Machine Learning algorithms to automatically replace the HDD which are about to fail with the hot spares. That is very easy thing to do for a person like me who work for a one of the best machine learning group in the U.S. I suggested my boss that we create the product like that but our feeling is that as the advanced file systems as HAMMER which can take the full advantage of SSD get more main stream there will be no need for S.M.A.R.T. at all as the expected lifetime of HDD will be over 50 years.
When I try to run it I get an xmessage which says
Error launching gmartcontrol: no suitable su mechanism found.
Try installing kdesu, gnomesu, gksu, beesu or sux first.

I'm using xfce, so which of the above is most appropriate?