I assume you mean momentary button connected to PWR+ and PWR- on the motherboard headers - yes, you could do that, but you'd still want to run ACC to your OPUS. That way, you just ignore the OPUS button pressing; when you turn on car, OPUS is on; when you turn off car, OPUS is off. Then, you can press the button to turn on computer, and turn off computer. If you forget to turn off computer, OPUS will kill power to it after OPUS shutdown delay.
I think it's silly to make something completely manual when you can have an automatic solution.
There's one other option. I use ITPS, but OPUS might be different - you could try tying the OPUS power switch wires and your own momentary switch. Depending on how the OPUS powers the computer, you may be able to use your momentary switch in addition to OPUS. THen if you forget to turn off computer before you leave car, OPUS will turn it off. Of course if computer is already off, OPUS will turn it on - because the OPUS doesn't know the current state of your computer.
If i recall, your goal was to only turn on the computer when *you* wanted it on so other people don't have to deal with it... Perhaps you could solve it another way - like only turn on the screen for the computer when you want it on?