The boot process of any machines is handled by its firmware, not by any OS.
So, this last should handle itself peripherals to be able to read/write any data from them.

For example, the mac mini has this kind of Firewire/SBP2 driver inside the OF, but Pegasos hasn't.
More over (but it's not the same thing), the mac mini OF can also make its internal harddrive visible
through the firewire port to any others connected machine by using the "Target Mode".
This special state of the OF transform your mac mini as an external Firewire harddrive case.

So don't expect that MorphOS and less Helios, to let you boot a Pegasos on an external firewire harddrive.

But the issue is not ends: the OS booting is in 2 steps, the first one is the loading and execution of the OS core bootloader, then the rest of OS itself through the famous startup-sequence.
The OS core can install Helios internaly and let you choose by an OF command option which harddrive contains OS data (MOSSYS: and SYS:).

In any cases, the OS core bootloader (the famous boot.img file) should reside on the internal harddrive (or maybe a usb hd/key?)

Note: Helios archive contains a device (sbp2.device) to mount sbp2 devices connected on the firewire bus and makes them visible on your Ambient screen. But there are nothing yet to expose your partitions on the bus (a sort of target mode, but with an usable OS in same time).