It's rare to find demos that support addons, and rarer still to find addons made for them, but here we have it folks: the first Dragon Age 2 Demo addon by Gibbed unlocks Character Customization and the Difficulty Level menu. UPDATE: A second addon was released to unlock inventory as well.
I've verified that it does indeed work, but you'll quickly see why these features were disabled in the demo - as I was adjusting the sliders, there was a noticeable flickering with my character model. Unfortunately, the first couple of presets (the default Hawke looks) are still locked from modification, but all the non-default presets are modifiable. Also, the demo was noticeably more difficult on Nightmare difficulty.
It enables the appearance customization and other junk in the character generation screen. Note: to customize appearance, slide the preset slider over a few times (you can't customize the default faces for some reason).
Another note, it also enables the difficulty selection after you complete character generation. Nightmare demo, anyone?
To open the chargen.7z file, you'll need an unzipping program like 7-Zip (freeware). Within, you'll find a folder named "packages". Extract it all into the demo directory, which should be located in: My Documents\Bioware\Dragon Age 2 Demo\ (which you'll notice also has a corresponding "packages" folder).
Thanks to Ploppy for bringing this to our attention.