Normally, to boot off an external drive you need to either select it as the boot drive in the Early Startup Menu (KS 2.04+ only) or use Degrader to force a boot from DF2:, which would require an already booted system, a catch-22.
However, with no internal floppies hooked up, I think the first external floppy will default to be DF0:. If not, then the system will try to boot from DF1:. At least, that's how it works on the A1200.
So yeah, you should be fine with an external drive :-)
