Without the original os, the only practical way (free of charge) is to flash via serial console, others use the Strongbolt install cd to flash with and then back out before it tries to install Strongbolt. I have never had to serial console flash as all units I have ever flashed either had the original os installed or later on I used the Strongbolt installer cd to just use the flash update portion.
I had purchased Strongbolt for my first raq xtr a few years ago. I have since used the Strongbolt cd to flash a raq 550 and a raq 3. In order to use theirs, you have to boot the cd on a computer that has either a 3com or intel 10/100 nic and it sets up the nfsroot environment automatically to allow you to boot over the net. Once booted, their installer checks the rom version and does the update semi automatically, it's their installer and automatic rom flashing utility that you pay the money for, not the os itself. I paid 20 bucks for it but actually have Strongbolt running a few XTR's I use as fileservers. Basically a cobalt with any of the roms will boot a kernel over nfs, in fact you could have an entire linux os in nfsroot if you had an nfs server to set it up, you would then boot from the net.
For serial flashing:
http://wiki.parvi.org/index.php/ROM_Flash_without_OS_Guide