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I have a table with an identity column and I want to reserve a block of ids which I can use for bulk inserting, whilst allowing inserts to still happen into that table.

Note this is part of a bulk insert of several tables, where those other tables relate to these ids via an FK. Therefore I need to block them out so I can prepare the relationships beforehand.

I've found a solution which works by taking a lock on the table in a transaction and then does the reseeding (which is pretty fast). But it looks a bit hacky to me - is there a generally accepted pattern for doing this?

create table dbo.test
(
 id bigint not null primary key identity(1,1),
 SomeColumn nvarchar(100) not null
)

Here's the code to block out (make room for) some ids:

declare @numRowsToMakeRoomFor int = 100

BEGIN TRANSACTION;

 SELECT MAX(Id) FROM dbo.test WITH ( XLOCK, TABLOCK ) -- will exclusively lock the table whilst this tran is in progress, 
 --another instance of this query will not be able to pass this line until this instance commits

 --get the next id in the block to reserve
 DECLARE @firstId BIGINT = (SELECT IDENT_CURRENT( 'dbo.test' ) +1);

 --calculate the block range
 DECLARE @lastId BIGINT = @firstId + (@numRowsToMakeRoomFor -1);

 --reseed the table
 DBCC CHECKIDENT ('dbo.test',RESEED, @lastId);

COMMIT TRANSACTION; 

select @firstId;

My code is batch processing blocks of data in chunks of about 1000. I have about a billion rows to insert in total. Everything is working fine - the database isn't the bottle neck, the batch processing itself is computationally expensive and requires me to add a couple of servers to run in parallel, so I need to accommodate more than one process "batch inserting" at the same time.

You can use procedure (introduced in SQL Server 2012):
sp_sequence_get_range

To use it you need to create a SEQUENCE object and use it as a default value instead of IDENTITY column.

There is an example:

CREATE SCHEMA Test ; 
GO 

CREATE SEQUENCE Test.RangeSeq 
 AS int 
 START WITH 1 
 INCREMENT BY 1 
 CACHE 10 
;

CREATE TABLE Test.ProcessEvents 
( 
 EventID int PRIMARY KEY CLUSTERED 
 DEFAULT (NEXT VALUE FOR Test.RangeSeq), 
 EventTime datetime NOT NULL DEFAULT (getdate()), 
 EventCode nvarchar(5) NOT NULL, 
 Description nvarchar(300) NULL 
) ;

DECLARE @range_first_value sql_variant , 
 @range_first_value_output sql_variant ; 

EXEC sp_sequence_get_range 
@sequence_name = N'Test.RangeSeq' 
, @range_size = 4 
, @range_first_value = @range_first_value_output OUTPUT ; 

Documentation: sp_sequence_get_range