Install 10.6 on Firewire disk (using 10.7)?

Hi all

I have just installed 10.7 Lion from 10.6 Snow Leopard on my MBP. I have an external FW800 drive that I would like to install 10.6 on in order to restore an old user account from a backup, which contains drive tools and utilities. I would like that as a seperate Target Mode boot disk.

However, when I insert my old Snow Leopard install disk it says "You can't use this version of the application 'Install Mac OS X' with this version of Mac OS X".

So, how would one go about installing a previous version of OS X on an external disk using Lion? Is it even possible? Would I have to use another machine running Snow Leopard in order to install Snow Leopard?