How can I hack the Bootcamp plist (or other) so it will allow me to select and install a Windows 7 ISO.
Currently it's minimum Win OS is set at Windows 8 + but my DVDs, licence and backups are essentially all Win 7.
El Capitan 10.11.6
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.plist hacks are pointless. Usually, the OP wants to create a USB Windows installer, but the Mac requiring the hack can not boot and install Windows from the created USB installer. This is why Apple normally prevents the user from creating the USB installer in the first place.
Furthermore starting in 2015, Apple starting shipping Macs with the BIOS removed from the firmware. Windows 7 require a BIOS boot method. So, if you have 2015 or newer Mac, you probably will not be able to install Windows 7.
If the minimum Windows is 8+, then Apple has not provided the necessary drivers for Windows 7. In other words, you usually can use the Windows 7 drivers to install Widows 10, but the Widows 10 drivers probably will not work with Windows 7.