Is it possible to dual-boot Snow Leopard on a Mid-2012 MacBook Pro running Sierra? I have the DMG file so maybe I can make a bootable flashdrive?
Unless I'm totally missing something from your question, no mid-2012 MacBook Pro can run a version of OS X earlier than Mac OS X Lion 10.7.4.
More specifically, each of the three mid-2012 MacBook Pros initially shipped with Mac OS X Lion 10.7.4 and the firmware does not support any prior version of macOS. On the positive side they do all support macOS Sierra.
If you have need to dual boot into an older OS, then it will have to be any version from Mac OS X Lion 10.7.4 and above.