In my experience, support for relative paths in file URLs is inconsistent. Some tools seem to support them, other tools don’t. For example, is file://c:/some/path relative or absolute? On Unix, it’s “obviously” relative, and on Windows it’s “obviously” absolute. I wouldn’t trust how a tool would interpret this without experimenting, so IMO it’s risky to assume that a locket and an installer will interpret the URL the same way.