For Macs, you can find the autoupdate file that you need to modify by

right clicking on Opera Developer App -> show package contents -> Contents -> MacOS

Following pigliacci's instructions worked for me. I was able to find the 40.0.2296.0 exe file here: https://www.opera.com/computer/thanks?partner=www&par=id%3D39772%26amp;location%3D412&gaprod=operadeveloper

Just make sure you have your internet disconnected before you start the install so it doesn't auto-update before you have a chance to change the operaautoupdate.exe filename.