We have always been able to get dessert boxed up - my DM will seldom finish a dessert after a full meal but being on the dining plan we ask for it to go. The only time they don't want to do it if it is something served hot - or with ice cream.
I have read that if you ask nicely, a CM might be willing to get you a slice of the grapefruit cake. No guarantee, of course. SR closes early too - 4ish? - so that may be a consideration too.
I agree with making an ADR during a off hours for dessert.
Generally speaking, no.They are not equipped for takeout..Occasionally a guest has come back and posted being able to get an item as take out, but that is not the norm