FitterBase::minimize() calls outliersContributions() and then removes the largest meas and ref outliers. We want to know more about what those outliers are (beyond the TRACE-level log messages), which we can do by adding a config option (outputOutliers?), a butler type (jointcal_outliers? though there will be a lot of files (one per inner minimize step, so this would get unwieldy quick), and two C++ methods to write that output (writeMeasOutliers, writeRefOutliers?).
Whether these really need to be butler objects is a good question: a single jointcal run could potentially produce hundreds of these files.