You can try that simply by yourself: make a copy from a working Yosemite on a supported newer Mac with Carbon Copy Cloner, transfer it to your unsupported older Mac and replace the Boot.efi in /System/Library/CoreServices/ as well as /usr/standalone/i386 with the version from here. Works with 10.10.3 - just tested with a late 2012 Mac mini and a late 2006 MBP. It's of course not the best way to install Yosemite ....
Thanks atvusr.
I can tell from your earlier posts that you are always very helpful.
I tried your suggestion but unfortunately didn't work for me. I am sure nothing wrong with your suggestion, but rather what I said earlier, that the most likely problem is something specific to iMac 5,1.
I usually get stuck at a black screen with an apple in the middle, and this follows by either automatically reverting back to the original partition, or it shows an empty progress bar for endless hours.