The only machine that had DDR memory and was able to boot into OS 9, was the PowerMac G4 (FW 800 signaled OS 9's death in that case, due to the PCI-based USB controller).
Apple started disabling OS 9 bootability in the eMac with the last series of SDR machines.
So if it's a SDR eMac, it either came with a OS 9 bootable partion -- or didn't.
Don't know too much about the non-OS 9 partition SDR machines and the capabilities to boot OS 9 -- it was too big a mixed bag for a one-liner explanation.
But, if it's a DDR eMac -- it won't work anyway. All new chipset switch from the SDR PowerMac chipset to the new Intrepid-base DDR iMac/iBook/PowerBook G4 chipset. Which Apple never wrote a OS 9 rom for.