Can targets (used for registering point clouds) mounted on a building be used to increase the accuracy of a point cloud generated from UAV imagery?
If you're using photogrammetric methods for producing the point cloud, targets won't necessarily increase the accuracy of the point cloud. What they are useful for is taking measurements from your point cloud or scaling/georeferencing your point cloud data to be representative of real-world measurements.
If you have access to a Total Data Station or other piece of equipment that can help you establish the location of your targets on your building relative to your grid coordinates (whether you use an arbitrary grid or one based on map coordinates with the TDS back sighted to true north), you can scale your point cloud dataset to your grid and incorporate other datasets.
Targets can also be used to stitch photos together if your process requires manually relating photos to one another. If you want a more automated methodology for point cloud creation you can check out the photosynth toolkit (http://www.visual-experiments.com/demos/photosynthtoolkit/)