I have a Giant Loop "Klamath Tail Rack Pack" on my Trident 660. I strapped it to the pillion seat. It's on the small side, so I wouldn't call it "luggage." But it looks and works great!
Former FJ-09 rider here. At first, I used a tourmaster tail bag that was held on with straps. It was very secure and held enough for an overnight trip. Eventually I got a rack and some givi hard panniers that could hold way more.
A number of companies make rackless luggage. They are found in the ADV/Dual Sport would, not sure if they will work on a MT-09. Go to the web site for Rocky mountain ATV or Chaparral Motorsports you should be able see if you can find something that might work.
A backpack and some paracord can be an effective solution. Learn a few basic knots and find some spots you can attach to...you can have a secure bag on the bike and a comfortable bag off it.
Tank bags are one of those black magic things I'm still hesitant about. What's your experience? Any scratches on the tank?
Edit to add: I usually wear a backpack for clothes and stuff for a few nights going somewhere, but it gets a little uncomfortable after an hour or so. Ideally I'd like to not have to wear the backpack at all but I guess at the very least a tank bag would lighten the load.
Kriega makes great bags! I use a US-20 Drybag from them and absolutely love it. Easy to put on and take off of my KLR and I'd trust it to keep anything dry. It's also easy to move around to other bikes, especially if you get an extra set of straps.
Whatever you do, make it safe. Was headed from Lassen National Park when a bike about 1/4 mile down the road went down. He had a pack strapped to his seat (no rack). Loose ends got tangled in the chain and pulled the whole thing into the rear wheel.