I was at Boulangerie last year. I personally was not on the dining plan. The group in front of me was. They picked pastries from the cases and walked down to the cashier, who told them you couldn't get pastries for snack credits. The group walked out and left the pastries, which were then put back in the case.
I would ask before trying to get Boulangerie pastries with snack credits.
As for the other stuff, reports from the field seem to indicate that in 2008 a snack credit must be used for a single-serving size item. A rice krispie treat would probably qualify. I don't know about the beignets; if they're sold in servings meant for one person, perhaps.