Can spent grains be fed as a primary diet for layer hens?
Nope, sorry i don't. Personally i would also give a proprietary feed in order to ensure that they get all the vitamins / minerals and also provide some oyster shell in a separate feeder - layers need more calcium than non-laying chickens.
I would not imagine it would make too much difference. Do bear in mind however that layers feed provides all the necessary vitamins / nutrients necessary, so anything else you give them should (I seem to recall reading on BYC before) account for more than 10% of their diet.
I think the bottom line is if they prefer the grains to layers feed, then only give as a treat or less than around 10% of their total feed per day.