If your time is valuable to you, then it would really pay to pick up a second PS3, a second copy of U3, and a heap of controllers, and perform split-screen Hunter boosting by yourself.
Doing so allows you to take kills as both Heroes and Villains - contrary to the beliefs of some people, treasures do get dropped when killing Heroes... but they're only dropped when the Hero is killed, not when they're downed. For this reason, I usually performed stealth or explosive kills on them.
There were very few games where I had less than 5 treasure drops, and in some games I was getting up to 9 drops. I managed to get my last ten treasures in about 25 games of Hunter... the biggest gap was for my penultimate treasure, which took 9 games (the final treasure only took 3 games!). 25 games is 12.5 hours, and I was expecting about 10 hours per treasure... so I consider the investment in the second PS3 money well spent (I already had two copies of U3).
Other details (which are all personal experiences and presented as a record of what I did, rather than rules):
I set up a brand-new account to boost against... once the first one got to Level 10, I created a new one. It felt like the lower levels yielded treasures more often. One account was local (Australian) and one was US.
I did use a hat (Elena's bunny ears) and a taunt (one of the Carltons) bought from the PSN store.
I actually found it easier to boost with four controllers than with two - you can get a really nice stay-alive rhythm going!
I left DLC on all the time, but flipped Hat Stats on or off depending on matchmaking collisions.
In some games I'd get no treasures from killing Villains at all... but then the Heroes would drop more frequently (and vice versa). Numbers seemed to average (as noted above) to around 5-6 drops per game.
"Rare" treasures came from Heroes and Villains.
One of the latter treasures - the [Vagabond] Tibetan Snuff Bottle - was picked up about 6 times soon after its first appearance, making me think that there may be a time or date component to the drops.
Easiest maps: Museum (by far the easiest for Hero killing), Chateau (set the Villain spawn to the interior area above a chest), and Fort (actually fun for both).