After much trial and error and lengthy encryption processes, I have found a solution. Note that for this test I used a USB-attached 160GB hard drive, so speed was pretty slow to begin with. Be sure the drive to be encrypted is either already blank or has been fully backed up to another device. This process will completely delete any files you had on the drive.
Install and launch TrueCrypt. Select to create a new volume and select your options (ie. encryption format such as AES or Twofish) and select the drive you plan to encrypt. Note that the OS X drive selection window does show the volume names you have assigned to each of your connected drives, so it's fairly easy to choose the correct one. Finally, choose to create the volume as FAT formatted.
After a lengthy process (my 160GB USB drive took around three hours), you will have an encrypted drive with a single FAT-formatted volume. Now you can launch Disk Utility and use the Erase> option (do not use Partition, or you'll overwrite the encryption) to change the volume's filesystem to HFS+ as you normally would for a simple reformat.
That's it! You'll then have your HFS+ drive which requires launching TrueCrypt and entering your password in order to unlock and mount. Attempting to mount the drive directly in OS X without going through TrueCrypt will tell you the drive is unreadable (as it should, since OS X itself cannot read the encryption).

