Is it possible to Bootcamp Windows 8.1 using USB on Late 2011 MBP?
Here are the steps I use to install Windows without needing to burn to a DVD or flash drive. Keep in mind that the final product will NOT be a virtual machine:


Install VirtualBox from http://download.virtualbox.org/virtualbox/4.3.28/VirtualBox-4.3.28-100309-OSX.dmg. Make sure it installs the command line tools.
Once installed, open it and create a new VM by clicking "New" on the top left.
Select the type to be "Microsoft Windows", and select the version as "Windows 8.1 (64 bit)", then give it a name of your choosing. Click "Continue"
Click continue again (it won't matter how much RAM you give it for this process).
Select "Do not add a virtual hard drive". We will be creating one on our own for our purposes. Click "Create", then "Continue".
Now we have to open the Terminal. Type the following command: diskutil list. From the list of disks, make a note of the disk identifier in the last column that belongs to the 30GB partition you created for Windows. We will need this in the next step
Run the command sudo chown $USER /dev/disk#s#. Replace disk#s# with the identifier from earlier. Enter your password and push return. Also run sudo chown $USER /dev/disk#s1. Use the same identifier, but replace the last number with a 1. Also run sudo chown $USER /dev/disk# using the first half of your identifier (minus the s# part).
Unmount the disk by typing diskutil unmount /dev/disk#s# again, using your identifier.
Now we have to create the disk for the VM to use for installation. Run the command VBoxManage internalcommands createrawvmdk -filename ~/RawWinDisk.vmdk -rawdisk /dev/disk# -partitions 1,#. Replace "disk#" with the appropriate first half of the identifier from earlier, and replace the second # sign with the last number in your identifier.
Unmount the disk again diskutil unmount /dev/disk#s#, using your identifier.
Now you should have a ".vmdk" file in your home folder. Go back into VirtualBox, select the VM you created, and click "Settings". Select the "Storage" tab, click on the "SATA" controller and click the second "+" sign directly to right of the controller (the hover text is "Add Hard Disk).
(Note: Due to a fairly old bug, you can't command+tab away from the selection window we're about to create without VirtualBox bugging out and requiring a restart of the app. Try to complete this in one step) Click "Choose existing disk", and navigate to the VMDK we just created, which should be in your home folder. Click it and select "Open". Now you should see the newly added disk just below the "SATA" controller.
Click on "Empty" just below the "IDE" controller, check "Live CD/DVD", and click the little CD icon next to "IDE Secondary".
(Note: The same bug applies here. Try to do this in one step.) Click "Choose a virtual CD/DVD disk file...", navigate to your ISO, and open it.
Start the VM and install windows on the 30G partition.
When the installation is done and you're logged in, shut down Windows and you should see the disk created by Windows appear on the desktop. You can get the Boot Camp drivers for your laptop from https://support.apple.com/downloads/DL1720/en_US/BootCamp5.1.5621.zip. Put this on a flash drive formatted as ExFAT or FAT so Windows can read it.
You should now be able to reboot into Windows by holding alt down, or selecting the disk in Startup Disk preferences.


This is a long process, but I believe I've covered all the steps involved... Let me know if anything goes wrong!