To mount a partition:

  do shell script "diskutil mount PARTITION_UUID"

To unmount all partitions:

  try
  do shell script "diskutil unmount PARTITION_UUID"
  end try

  try
  do shell script "diskutil unmount NEXT_PARTITION_UUID"
  end try

Just compile those as self-contained apps, drop them in your dock and you're good to go. It's pretty manual stuff, I have to remember to launch the appropriate script at the beginning and end of a task that involves the HDD, but it's not much of a problem, since I don't use it that often. Meanwhile, you get a silent MBP, with a smidge better battery life, since the HDD isn't spinning needlessly.

Looking into auto-mounting the partition when a symlink is accessed, got a SuperUser question going here:

http://superuser.com/questions/228028/mount-drive-when-symbo...