IMHO per-file linkage to a shared location is confusing, which I think explains why most SCMs chose to adopt a paradigm allowing sharing in the directory level (if at all). In addition to the example you bring of PlasticSCM, I have experience with git
which provides this functionality as submodules, again requiring you to put the shared files in a separate directory, and forcing a less-confusing hierarchical relationship between the shared part (the submodule) and the projects using it.
Unfortunately, it seems you fell in love with a feature not many need or use. It might be that your needs are so unique. But you should carefully consider whether your requirements are indeed unique, and perhaps make the small sacrifice of giving it up for more mainstream development conventions (in this case, putting the shared files in a separate directory), which will allow you to use state-of-the-art tools.