The full file spec is not the only method by which MicroStation resolves references. In addition to full file spec, you can make use of the following:
When MicroStation opens a DGN file, it searches for DGN files that contain attached references as follows:
1. If the file specification contains a custom configuration variable, it searches that directory, using the relative path, if any.
2. It searches a list of directories which consists of all of the directories for the parent files of the reference. If the reference is attached directly to the master file, the search path consists only of the directory of the master file. If the reference is nested, the path consists of the directories of each parent file, starting with the most deeply nested. The directories searched include the relative path, if any, in the file specification.
3. It searches the directories specified by the configuration variable MS_RFDIR, again retaining the relative path, if any, in the file specification.
4. It uses the full file specification in the reference attachment, if present.
Note that the full file spec is the last place MicroStation looks to resolve the attachment.
The above information is found in the "Locating Attachments" topic of the help.