Hi all. First, huge thanks to all the stalwarts here who are helping us keep our machines completely up to date — fantastic work guys, and much appreciated.
Just a report on an issue I had getting the El Capitan 10.11 release installed on my MP 1,1.
I made a USB installer following the Hennessie method, exactly as I did successfully with Yosemite previously but using the official 10.11 installer. But, I couldn't get it to boot the mac. It wasn't visible in the Startup Disk pref pane and holding C at boot didn't work. I have unflashed PC Radeon 5770 graphics so no boot screen option. This was all done from my Yosemite system. I was ready to give up and wait until I could borrow a supported mac.
Then, I booted into an old El Capitan beta partition (installed using firewire target disk mode on the borrowed, but not currently available, supported MacBook Pro) and, lo and behold, the USB installer was now available in the El Cap beta Startup Disk pref pane. From there, I rebooted to the USB installer and El Capitan 10.11 installed like a breeze. I swapped the 3 boot.efi instances while in the Yosemite system and I'm now typing this in a brand new 10.11 install.