Gamebore do a very nice (although expensive) fibre wad that is very similar looking to a plaswad.Eley are trumpeting the 'improved' fobre wad in their cartridges.This is no more than reclaimed cardboard compressed paper in construction.
I have seen no test papers or reports about the performance of fibre wads in overbored barrels.I would have thought there may be obturation problems, but they seem to perform okay to me.