environment: any MBP/MBA having GPT (yes GPT, not MBR, because 10.11/10.12/Sierra/etc will not allow you to have MBP partitioned drive and installed on it, especially on wireless recovery mode). however if you don't have MBR drive partitioned - windows 7 will refuse installation - the only cure is a "hybrid" GPT/MBR configuration explained below.
prerequirements: you need 2 USB sticks = 1 formatted NTFS (because you might have no DVD drive anymore around) and created as Windows7 installation using unetBootIn, 1 formatted FAT32 for win32/64 MBP drivers to be stored onto with the help of boot-camp
step 0: all you need from the boot-camp these days is to get WindowsSupport folder downloaded into your USB stick FAT32 formatted one. trying to install windows 7 using boot-camp will complain about single only partition you must have on your HDD and will complain about installing ONLY from DVD disk (imaging the hassle as modern MBP/MBA has NO DVD drive embedded at all)
step 1: you have to migrate windows 7 installation from DVD drive/image into USB stick. the best way to do so is to use unetbootin. make sure you have NTFS formatted stick otherwise it will not boot installation after.
step 2: repartition your hdd drive using regular macosx disk utility (let it stay in GPT mode, it won't matter for you anymore).
step 3: download and install refind boot loader replacing original useless MBP/MBA pseudo-bootloader (you can always comeback to the original one by holding "alt" key during boot process). refind will let you choose what media to boot from including: USB stick with windows 7 installation image and HDD partition with already installed windows 7 (later)
step 4: download and install gdisk. it will require to append hybrid MBR partition emulation (without boot flag enabled!). you sudo gdisk with your drive device name. print list of partitions (command 'p') and remember the one you've created for future windows 7 installation. then switch into "recovery and transformation mode" (command 'r'). make hybrid MBR (command 'h'): specify windows 7 dedicated partition number (single digit), answer 'y' on place EFI GPT partition first, answer "default" on MBR hex code (just hit Enter), answer 'n' on set the bootable flag. answer 'n' on "use one to protect other". write table to the disk (command 'w'). if you specify bootable flag 'y' - you'll have a windows 7 usb stick bootloader failure (it will discover bootable win7 partition on your hdd and will try to boot from it instead).
step 5: plug windows 7 usb stick and reboot. you'll see 3 choices to boot produced by refind bootloader menu: "your macosx partition name", "boot windows (legacy) from Basic data partition" - this is your future windows 7 partition to boot from, "boot windows (legacy) from NTFS volume" - this is your NTFS formatted USB stick with windows 7 installation - run installation from it now. inside windows install select "advanced" installation and choose preselected partition by yourself. don't delete the partition but I'd suggest to format it within the menu. the rest of installation process will be relatively simple and standard. remove the stick once the windows will enter reboot state.
step 6: plug in fat32 formatted usb stick with WindowsSupport folder you've downloaded with the help of boot-camp application. execute setup from inside the folder and wait for all the drivers to be installed. reboot system and you are done with the 2 systems booting in parallel from the refind bootloader.