Mean while, I believe that you can already accomplish your stated objective. UR stores links relative to your urd file if the linked files are in or below in the directory structure.
See-
http://www.kinook.com/UltraRecall/Ma...ronization.htm
"Note: For files located in the same folder or a subfolder of the .urd file, when imported or synchronized, the path will be stored in the URL attribute as a relative path (relative to the location of the .urd file), allowing the .urd file and synchronized files to be relocated to different paths or drives intact."