There are people who get just as much satisfaction out of improving existing code as "feature" programmers get out of creating new features, both in the open source community and working for business.
If you identify the missing 10% and make it visible to those people, unless your process makes contributing painful or your code base is painful to work through, chances are someone will be willing to work on it, especially if they are directly impacted by it.
As an open source project, the best thing you can do to encourage people to help with the code is to make your codebase clean and readable and let people know what the issues are.