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.