How tall can I go above ground with a concrete footing?
If properly reinforced and if the actual footing (which the 12" column isn't, unless you have no flare or flat pad on the bottom, which you should reconsider unless you have stunning bearing) is of adequate size to support both the imposed load and the weight of the longer column, it should not be a problem. 

But it's difficult to accurately assess such things from afar, or to be certain that it will be safe in your particular soils and configuration. A few hours of a civil engineer's time to establish exactly what would be safe is probably money well spent on peace of mind. 

This article might be inormative even if it's not all that DIY oriented. The sidebar suggests that the minimum cover (concrete between surface and steel) should be 3" on the bottom (earth) side of the footing, and 1-1/2 inches on the sides of the column, assuming you are using #4 bar which is pretty typical for small projects. So you might use 6 #4 bars in a 8-9" circle, bent outward on the bottom and tied together every 8-12" vertically. If you really want to be the oddball overkill homeowner you'll figure out a way to bend a bar into a spiral to tie the vertical bars together, just like the big boys do. The rest of us normally compromise with wire, other than the ones that hope fibered concrete is all the reinforcing they need.