Hmm, not familiar with this, but there may be a couple of options:
Is there a menu where that might be selectable? Maybe assign the 12V trigger to always on instead of zone 2?
The other option is the wall-wart solution. If the receiver or preamp has a switched plug on the back, you plug in a wall wart AC to DC power supply (12V nominal) and adapt that for your 12V trigger. Quality does matter, as some lower end power supplies say 12V and can vary from 6 to 20! The better ones hold more stable around 12V (10-18V), and the expensive switching power supplies hold a stable 12V. Some come with the proper plug on the end, but you usually have to solder on or splice in a new plug to make this solution work with your other gear.
Hopefully someone with this unit will chime in as they may have jumped this hurdle before.
I had same problem with my NR 906.The only way you can use 12 volt trigger in zone 1 is to connect the zone 2 trigger out with your zone 1 power amp and use macro on you remote to switch on and off zone 1 and zone 2. It works well for me.
I am an owner of an Onkyo NR1007 AVR and faced the same problem. I ended up using a smart power strip and a wall wart power supply. The AVR plugs into the master outlet and the wall wart plugs into a slave outlet. When the AVR is turned on, the slave outlets are energized and which powers on the wall wart and triggers my power amp. As a added bonus, I use a second slave outlet to turn on my external AVR cooling fans.