I already read your opinion of anti-leecher, and so I advice you to consider the implementation of ClientAnalyzer, by eMuleFuture's mod. It does not work on nothing concerning strings or names, but work on behaviour. The client that acts bad, has a reduction of score. I know it's very difficult to do a porting, but I thing it would be nice in the future to have protection from leecher.
BTW, is it possible to add the feature to open a file by double-clicking on it when it's fully downloaded?