Would it be possible to add a column select in the Weka classifier dialog to select a double column containing instance weights (optional)?
A number of the Weka classifiers can take advantage of instance weights in the learning: http://weka.sourceforge.net/doc.dev/weka/core/WeightedInstancesHandler.html.
This is particularly useful when dealing with class imbalance and I've succesfully used the following equation for generating instance weights:
Weight(instance) = size of minority class / size of instance class
Where the minority class will get a weight of 1 and all other (larger) classes will have a weight less than 1.