Bypassing Boot Camp to install Windows using a non-standard partition scheme
I have installed Windows 8.1 Pro via Boot Camp on my MacBook Pro Retina 15-inch mid-2015, with OS X Yosemite 10.10.4 and Boot Camp Assistant version 5.1.4. I can prove that this version of Boot Camp is no longer creating hybrid GPT/MBR partition scheme to install Windows 8 x64 on Intel-based Macs, and Windows is directly booted in EFI mode. Here are what I have tried:


Running Ubuntu on its installation flash drive, I ran sudo disk -l /dev/sda to check my local Mac SSD; results:
MBR: protective, BSD: not present, APM: not present, GPT: present Found valid GPT with protective MBR; using GPT Therefore Windows is not booting from and running on a disk with hybrid GPT/MBR scheme.
In Windows, C:\Windows\panther\setupact.log has one entry: Callback_BootEnvironmentDetect: Detected boot environment: EFI How to tell if windows 7 installer boot in EFI or BIOS?


Therefore theoretically Boot Camp is not needed to install Windows 7/8 x64, which support booting from GPT disks on UEFI(that is EFI 2.x)-based systems. But still it's better to use Boot Camp to create the Windows Installation flash drive and download drivers for Windows. The guide: How to install Windows 8.1 on Mac without Boot Camp To sum it up:


Partition your disk as you wish in Disk Utility in OS X; just format your desired Windows OS and data partitions as FAT32.
Since Retina MacBook Pros do not come with a SuperDrive, you need to create a Windows Installation flash drive from the ISO file. Please do this in Boot Camp Assistant. I have tried run dd command on Mac's Terminal to copy the ISO, but the created flash drive is not bootable on UEFI Macs: the Apple boot manager simply does not recognize the flash disk.
Plug in the flash drive, restart the computer and hold option to open Apple boot manager. You will see a yellow drive called "EFI Boot", which is the Windows Installation. Continue the installation as normal, and format your Windows OS partition as NTFS when asked.
Whenever the computer restarts, you need to hold option key and select "Windows" drive on your local SSD to continue installation. Finally go into the system and drivers from Boot Camp will automatically install.


You should keep a Time Machine backup before these operations. Even if things really mess up, you can just create a OS X Yosemite installation flash drive, re-format the SSD to one partition and re-install OS X. How to make a bootable OS X 10.10 Yosemite install driveThe newest version of Recovery HD partition will automatically come back. This worked successfully when I wrongly operated the disk in Ubuntu.

Hope this works!