I believe standard road derailers accomodate 27 tooth cog at most. You would need a long cage derailer to go higher.

With a 12% AVERAGE grade you are looking at a killer climb and most likely need a triple or put a mountain bike cassette and derailer on so you could at least get 39/34. Any shimano 9 speed mountain bike cassette will fit a 9 speed shimano compatible road hub/shifter. The spacing is identical, so it's just a matter of swapping cassettes. You could try it w/ a standard derailer to see how it shifts but I'm almost positive it's not recomended.

I did torture 10,000 with a 39/34 and was managable. It was 13000 feet climbing over 100 miles.