How to "pathconvert" under Maven?
Answering my own question:
Use ${file.separator} instead of slash and watch out for bug MEXEC-81 which was fixed in 2010 by git commit 6e8be6881fe50714a00509f8f106e21d50d606a6 (svn: 12372) which was where quotes and backslashes were misinterpreted.
In the case of adding a dir to the library path, you are better off creating a new artifact with its own pom, installing it in your local repo and adding it as a dependency in the project.
Maven and Native Libraries: JDIC and java.library.path UPDATED goes into more detail about getting a native dependency into the library path.
Using native dependencies inside Maven Is another approach.