You can dry-hop in kegs but from what I've read on HBT & Probrewr, dryhopping below a certain temp (i think it's around 60f degrees) takes a while to get same results - many saying dryhopping at fridge temps not nearly as effective as dryhopping at fermentation temps ... not to say you couldn't dry hop in keg at room temp for a couple days before you put in your fridge.