Several months ago I contemplated getting an older, powerful Mac for a backup. I discovered the issue you talk about yet I saw older computers running Mavericks and Yosemite. I found out how they were doing it and I will post the info here with the following disclaimer. I didn't write it. Didn't try it. Don't know squat about it and it may not be what you need or it may not apply to you. But I'm hoping it will point you in the right direction as a means to salvage your Mac rig. Here it is:
How can these Macs run Yosemite?
Apple does not support OS X Yosemite on the original 2006/2007 Mac Pro and 2006 Xserve. These 64-bit Mac Pros and Xserves have EFI32 firmware and can't natively boot OS X Yosemite "out-of-the-box", but are easily capable of running it with a different boot loader. Once a new boot loader is used to launch the native 64-bit Yosemite kernel, OS X 10.10 boots and works normally exactly as it does on a 2008 Mac Pro or newer with EFI64 firmware.
Background:
OS X 10.6 Snow Leopard was the first OS X version with optional support for a 64-bit kernel, allowing booting either with a 32-bit or 64-bit kernel. However, Apple did not support booting the 64-bit kernel in Macs that shipped with EFI32 firmware, even if they had 64-bit processors capable of running the 64-bit kernel. When Apple dropped the 32-bit kernel entirely from OS X, starting with OS X 10.8 Mountain Lion, EFI32 Macs no longer had an Apple-supported mechanism to boot newer OS X versions.
The last version of OS X officially supported by Apple on the original 2006 Mac Pro MA356LL/A (MacPro1,1), 2007 Mac Pro (8-core) MA1186/A (MacPro2,1) and Xserve (Late 2006) MA409LL/A (Xserve1,1) models was OS X 10.7 Lion, and then only when booted with a 32-bit kernel due to their EFI32 firmware. However, these Macs were the most powerful and upgradable EFI32 models and have a 64-bit architecture, so the user community has been keeping them running with 64-bit kernels and newer OS X releases using a variety of methods.
