Hi Darren,
I was able to calculate the volume average velocity by first calculating the volume of each cell (see Python calculator and programmable filter - KitwarePublic), then extracting an Excel-file with all data, and then I just did sum_i(volume_i*magnitudeU_i)/sum_i(volume_i) for all cells of the mesh. Might not be the ideal way, but it works.
All of the above is done in ParaView and Excel!
For the shear stress I will start trying now 