This is what I found on ozclothnappies:
Some people choose bicarbonate of soda, borax, or vinegar to wash or rinse with, however this is not recommended by some nappy manufacturers: please check first, as sometimes these substances may cause wicking, stink or even destroy your elastic or PUL.
I'd be checking with the manufactuers first as you wouldn't want to ruin any of your nappies.