Thanks a lot for looking at this guys. Some good news, followed by some sad news, followed by some more good news...
The good news is, disabling the "virtual installation drive" of the HP Gen8 Microserver worked and the boot process is now successful, so this issue is device specific. The sad news is, once the installer loaded, I realised I accidentally wrote the same boot image on two USB drives. Suffice to say, (and this is truly embarassing, but here we go) the image in question was not a FreeNAS one... I couldn't tell as I'd never seen the FreeNAS booting process before, right? The final piece of good news is, FreeNAS does NOT try to write a configuration file to a physical disk during a live boot which I imagine should be read-only par excellence. :)