I have the same server HP DL180 G6, and you can create the 2GB lun for DSS to boot from.
I assume you are creating your raid configuration through the bios of the raid controller.
This is where you are going wrong, this bios for HP raid controllers is very simple and does not offer all the features. You need to run HP Array Configuration Utility (ACU)
ACU can create a 2gb logical drive for DSS, and then you can use the remaining space for a data disk.

If you google "smartstart-8.70B-0-x64.zip" You should find the HP SmartStart CD which contains the ACU utility to create the proper raid configuration.
http://h20000.www2.hp.com/bizsupport...4ef394e57dd739
If you don't like booting from cdrom or simply don't have one you can use the HP USB Utility "hpusbkey.zip"
http://h20000.www2.hp.com/bizsupport...reg_R1002_USEN

Creating the DSS boot lun on your hardware raid controller is the prefered method over booting from USB.