I think the restaurant waiters had little chance of finding a place in a lifeboat for two main reasons. First, they were male. Second, they were a long way from first choice when crew members were needed to man the boats - when seamen were not available, the likes of stewards and firemen were second choice as they had (or should have had) at least rudimentary training in evacuation procedures. A number of other groups, like the musicians, postal workers, bellboys, even the doctors, found themselves in the same unfortunate situation.
There were 35 waiters and 9 of their bodies were recovered - same as the overall ratio of lost & recovered. This makes it pretty clear that the restaurant staff were not confined below decks at the time the ship went down, though as Dave said it seems there was an effort to keep them out of the way in the early stages. They certainly were not locked in a room and forgotten about, as is sometimes suggested.