I sent Onyx an email by way of their feedback form in the Settings area on the device asking them to include the screenshot function as an option after a long press of the power button.
Received a reply not too long ago saying that they would be implementing that function in the 2.1 update.
This is great news on a couple of fronts: first, that they will be providing this much needed function; second, that they are continuing to work on updates for this device.
Of course, there was no word on when this 2.1 update would be coming out...
So, if there is something you want them to work on, it might be an idea to contact them through the feedback form on the device.