It almost certainly won't work, but bring along a MorphOS CD to test it in person to confirm.
Do you have any written info, or test results to support your opinion that it won't work? I would have guessed the exact opposite, and thought I had read about someone actually getting it installed and working, but I can't find it now, so I might be mistaken.
To the original poster, do as Matt_H suggests and take a MorphOS2.6 installation CD with you to check out the condition of the eMac before you purchase it and test it out. It does not take long to boot from the CD and see if the installation script begins. If the seller does not mind losing the MacOS installation on the hard drive (many sellers wipe the hard drive anyway), ask him/her if you can install the MorphOS2.6 time limited demo version to the hard drive. That way you will know for sure if MorphOS2.6 works on the 1.0GHz eMac before you make your purchase.
Good luck,