I seriously doubt the rear main seal is leaking. I also doubt the oil filter housing gasket is leaking either, but the oil pump pressure sending unit may be attached to the housing and if that is leaking, it would be a concern to me simply because these can fail and dump a lot of oil in a short time. Fortunately, they are pretty cheap and not to difficult to change, but they are also usually pretty reliable. They don't fail very often.
Monitor your oil loss. If you are not loosing a quart every 500 miles or less, then i would just check the oil level at every gas stop and add oil as needed. If you don't see amy oil loss, then its possible that what the dealer saw was just oil that dripped down as the oil filter was removed. The Odyssey is not prone to oil leaks, most of the time, when there is an oil leak, it is very minor.