Well the problem with a single tab key is that its not directional and StreetDeck is. You can use the arrow keys to move the focus between all the different controls in the skin, but it couldn't be a tab key since you have controls both below, above and left and right of the selection. The real problem comes in with lists since you can have a large number of items in lists, so even if you id each control to tab in a certain order to it, your still going to need a way to switch to the list and select from it. There are certainly ways to get around this, but to make it 100% tab controllable and usable while driving would mean compromising some of the touchscreen features.
That being said, we have recently sourced a control knob in similar styling to the griffen, but exponentially greater in usability in the car enviornment that we will be building support in for. I will do a video blog on it as soon as its ready.