I might be wrong here - but dont milk banks work by basically pouring all the milk ina big vat and babies are fed off that (I know its not exactly liek that, but you get the picture )
So dont all womans milk get diluted and strengthened by the other mums? So if one woman has something in her diet that may react with bubs - theorectically this will be watered down by someone elses milk.
And they babies wont be getting much milk anyway - as they are premmies etc - so I'm thinking the effect of food on the milk quality would be negligable.
Just my thought - not sure if this is correct or not.