var queryable = collection
    .AsQueryable()
    .Select(x => new P { IList = x.List })

private class Test
{
    public int Id { get; set; }
    [BsonRepresentation(BsonType.String)]
    public List<E> List { get; set; }
}

private class P
{
    public IList<E> IList { get; set; }
}

private enum E { A, B, C, D };