Is there a secure procedure to move the macports directory strucutre on a non-root partition or disk?
Use a symbolic link...
A symlink is basically like a file or folder shortcut but it's different in that that it acts as though it is the location it's linking to. To the operating system, a symlink and the actual location are no different.
What you'd do is copy the contents of /opt/local to the new location (including hidden files) then, from the /opt/ folder create a symlink.
The format for symlinks is:
So, the link would be something like:
Since you want to put it on another disk/partition you have to make sure it mounts on boot. If mac doesn't automatically mount the drive/partition in question on boot you'll have to manually edit fstab.
Here's the wikipedia article on symlinks.
Here's a link describing how to move the '~/' or 'Home' folder in linux using a symlink. It should work relative the same in OSX since they are both POSIX
Here's a link describing how to edit fstab on Mac.
SideNote: I heart symlinks. They make some things sooooo much easier.