Cool, I was quoted in the sticky from the Acadia forum about hub centric rings.
I don't know about the Traverse group, but there have been MANY posts(some with pictures) on the Acadia forum. Both the Enclave and Outlook forum had some members swap out to 20" or bigger aftermarket and OEM style wheels, however it seems the Acadia forum has the most active posts regarding the subject.
You are correct that there was, and sounds like still is, very limited styles to choose from. However there is hope. I personally, as had a few others, have contacted specific companies and inquired about custom drilling to the Lambda bolt pattern with good success. My suggestion to you would be to pick up a magazine called something like "Ultimate wheel and tire guide". I think I found my copy at Borders or possibly the magazine stand at our local supermarket.
I browsed through this and marked off wheels that looked appealing regardless of sizes stated. From there I proceeded to visit the listed manufacturer website and looked for what they will call "blanks". These are rims that have yet to be drilled. I called the manufacturer to see if it was possible to drill our bolt pattern(6X132) in the desired wheel and made purchasing decisions from there. I ended up with a 20x8.5 Bazo Forged B359 Monaco wheels that were custom drilled. Look up my profile name on the Acadia forum and browse through my posts, you should find some with pictures.
As for your question on the Wheel Adapters, I believe them to be safe as many have been sold and used over the years. However we determined on the Acadia forum that it really wouldn't resolve the problem since the adapter would have to add so much thickness to the application, any wheel you could find in the new bolt pattern would likely end up pushing the wheel/tire out past the fender line due the necessary offset. You can judge for yourself if you'd be okay with this look, but IMO it is horrendous. Here is the link to the thread so you can read yourself.
http://www.acadiaforum.net/forum/index.php?topic=3493.20
I hope all this helps in your search. Be sure to post pictures if and when you install the new wheels. Curious to see how a Traverse would look with 20" or bigger as my wife has an '09. :thumb: