Extracting a field from RESTORE HEADERONLY
This works for me.

SELECT BackupStartDate 
FROM OPENROWSET('SQLNCLI',
                'Server=MARTINPC\MSSQL2008;Trusted_Connection=yes;',
'SET NOCOUNT ON;SET FMTONLY OFF;EXEC(''
RESTORE HEADERONLY 
FROM DISK = ''''C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQL2008\MSSQL\Backup\DB1.bak''''
'')'
) 


The ad hoc distributed queries Option needs to be enabled. Or if you don't want to do that you can set up a loopback linked server and use that instead.

EXEC sp_addlinkedserver @server = 'LOCALSERVER',  @srvproduct = '',
                        @provider = 'SQLOLEDB', @datasrc = @@servername

SELECT BackupStartDate 
FROM OPENQUERY(LOCALSERVER, 
               'SET FMTONLY OFF;
               EXEC(''
               RESTORE HEADERONLY 
               FROM DISK = ''''C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQL2008\MSSQL\Backup\DB1.bak''''
'')')