Now that my trip is complete, I wanted to weigh in on my experience. Earlier in September I used a Swiss boarding pass (departing terminal 5) to enter security at Terminal 1 so I could check out the Polaris lounge. The lounge was incredible and I highly recommend it. I then walked over to G in Terminal 3 and caught the bus from G17. They take a break from about 2-245 for their shift change/lunch. We caught it around 1:40 and had just missed one. We waited maybe 10 minutes max. They stated any passenger using Terminal 5 can use the bus. Definitely is not limited to American Airlines passengers or One World.
Polaris lounge attendant tried telling us we might be denied but we decided to try it anyway and it was a terrific experience. The bus ride provides great views of all the airplanes on the tarmac! Highly recommend.