If you are using migrations you should be able to simply modify code based migration to use newsequentialid() as DefaultValueSql.

[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]

public override void Up() {
    CreateTable(
        "SomeTable",
         c => new {
            Id = c.Guid(nullable: false, defaultValueSql: "newsequentialid()"),
         })
         .PrimaryKey(t => t.Id)
    );
}