Hi Max:
Just for me to understand better: did you apply source orientation after averaging time series across the atlas parcels? If yes, then the orientation is not informative. As a side note, averaging sources across parcels is computationally effective for PTE NxN computations but will be detrimental to sensitivity to higher-frequency components, which will average to zero across the relatively large surface areas of the parcels.
I would recommend to 1) extract source orientations 2) average across parcels and 3) compute PTE / normalized PTE. Alternatively, 2) could be computing exhaustively across all sources and then average PTE values across parcels, if computationally tractable.
I hope this helps.