If you're using three ounces of hops, make sure you use a huge bag, or several small ones. You want the hops loose in the bag(s), and not packed in, so that the beer can contact/permeate all of the hops.
I dryhop in the keg all the time. Sometimes I leave the keg at room temperature for 5 days before chilling (to get the most out of the dryhops- it happens faster at room temperature), sometimes I chill right away. If I chill right away, it takes more like 14 days to really get the dryhops aroma out of the beer (due to the colder temperature).
Either way, I love the results and dryhop in the keg often! The co2 doesn't seem to matter at all. The only thing I would do is make sure to not carbonate by "burst carbing" the keg by shaking/purging- as each time you purge the keg you'd be blowing off the nice hops aroma you're trying to develop. Otherwise, putting it on the gas is fine.
Broken Leg Brewery
Giving beer a leg to stand on since 2006