A long time ago, I added a Weka meta-classifier for parameter optimization called MultiSearch. In contrast to GridSearch, which forces you to optimize two parameters (hence grid), this scheme allows you to optimize an arbitrary number of parameters. However, it does not allow you to extend the search space like GridSearch.
Today, I moved this code into a separate project on github and turned it into a Weka package, making it easier for other people to use it as well.
You can find the code and package archives here: