What that application does? Maybe we can find another approach. If it's about a form's control validation, there are solutions with onkeyup and onblur.
This part of the system is extremely complex because there is no real standard to keyboard input, every browser on every OS does whatever they want... You just can't be sure what was actually pressed by the user, example: The keydown of arrows gives you letters... WTF.
After working on the feature for over a month, I finally managed to map the entire keyboard keys correctly, by cross examining values received from both keydown and keypress events.
So I don't think it's a good idea to look for a difference approch at this point... Not for a browser with less than 2% coverage.
In other words, if I won't be able to find a bypass to this misfiring issue, we will simply drop Opera support.
Also, I doubt if onkeyup will act differently than onkeydown and onkeypress.