I dont think there is another way to trigger it without using another app and without rooting your phone. You can whip up an app that calls the intent to start quickmemo. It would be really 1 line of code. You can run this command from adb or a terminal emulator "am start -n com.lge.QuickClip/.QuickClipActivity" and it will start quickmemo, at least it does on my VM zv5 firmware. Just stick that same intent into a simple apk or widget or something and you're all set.
If you have rooted your phone, you can edit the keylayout files. I have changed mine so the search button behaves as if it were a camera button. You can change yours so another button acts as if it were the volume up, or maybe even both volume buttons. Then again, if you are rooted, you'll have quite a few more options for getting screenshots.