Security Update 2018-001 for Mac OS X El Capitan from Apple will break your MacPro 1,1 or 2,1. If you have OS X El Capitan up and running on your 2006/2007 Mac Pro with Pike's boot loader, you should consider disabling 'Install OS X updates' and 'Install system data files and security updates' in System Preferences > App Store so that OS X doesn't automatically install any future updates that may overwrite Pike's boot.efi.

MacVidCard's guide for installing El Capitan to a 2006/2007 Mac Pro using a second El Capitan-supported Mac.

rthpjm's Pikify installer script and accompanying files (and now there's an App!) can build a pre-patched OS X El Capitan installer with the Piker-Alpha bootloader for the 2006/2007 Mac Pro.

El Capitan branch of the Piker-Alpha bootloader - development thread.

Simply looking to download the Piker-Alpha macosxbootloader for El Capitan? Links:

This first post has now been updated with recent summarized information. UPDATE: Only one day later, 666sheep did it first!

Who's the first to get OS X version 10.11 El Capitan booting perfectly on a 2006/2007 Mac Pro (1,1/2,1)?

Peavy link ii mac os 10.12