I wrote a small hack/mod that lets a user link a post purely by the PK. It can go anywhere in your directory structure.
I haven't truly tested it completely out yet, but, for example, try the following urls.
www.domecanmar.com/gotothread.php?gotopost=91884 www.domecanmar.com/gotothread.php?gotopost=381 (error should occur)
You should see a favourites icon in the post extras section of show flat/threaded which can be used to link this document from another site. If the post happens to move boards, the script will find this and redirect the browser.
I've included in the zip file a small installation file, so it shouldn't be to hard to implement (changing 4 lines in 4 files)
Let me know if and what i should change. HTH
EDIT - please don't register a name or post in the forums! It's a copy of existing and my users may get emails