Like the others said, you can dry-hop in the primary. If your primary is a carboy and you're worried about getting the bag in and out (seems to be the point of your post)...well, you can always dump the hops in without a bag, and tie the bag around the siphon inlet to act as a filter when you rack to your bottling bucket. Or not even bother with that and just siphon carefully (I can't imagine leaf-size bits easily getting through my autosiphon).
The issue with doing secondary in a bucket is head space. When you primary in a bucket, the CO2 created by fermentation protects the beer from oxygen exposure, so as long as you don't mess with it too much this blanket of CO2 protects that exposed area (which is as big a surface as the mouth of the bucket). A 5-gallon carboy that is usually used as a secondary is used because the 5 gallons of beer will reach up into the narrow neck of the container, making an area maybe just a few inches across that's exposed to air, which is important because at this point the beer isn't producing much CO2. The big thing to worry about doing a secondary in a bucket, basically, is that it increases the risk of oxidation.
