I have an ECS motherboard and the BIOS has options for 1st Boot device as follows:
ATAPI ZIP C:
I don't have a zip drive to test it but with the last option set, it booted from a floppy in my LS120 drive. From this I would say, if your BIOS has the necessary boot option then give it a try. You would need to format the disk as a system disk though because just copying the boot files wont work and since I have never used a zip disk I don't know if this option is available when formatting the disk.
Having said all this I have to add that a boot CD would be easier and more universal.