Looking both at the high-level Stitcher class and the detailed demo in samples/stitching_detailed.cpp, you would have to work here:
detail::HomographyBasedEstimator estimator;
estimator(features_, pairwise_matches_, cameras_);
...
bundle_adjuster_->setConfThresh(conf_thresh_);
(*bundle_adjuster_)(features_, pairwise_matches_, cameras_);
In a sense, you already have computed the homographies frame-by-frame. Therefore you can skip re-estimating homographies for all frames. When you set the cameras to you guesses, they are used by the bundle adjustment as initial guesses.
However, it doesn't seem that the current stitching pipeline can update only the (k-)newest frame(s). So this approach would work if you record a video and stitch the images together finally. If you wanted to stitch continuously, you'd have to construct a more complex pipeline.