You could try the aftermarket exhaust silencers that slip into the tips. They're primarily used on the aftermarket cannon mufflers like the Apexi N1 style. They work pretty well on aftermarket mufflers, but not sure if you'd be able to fit them onto the rolled factory tips.
There is another type of silencer that I'm not sure is made anymore. It fits inline w/ the exhaust, right at a connection, it's flanged and is basically a throttle body. Back in the day, I think they were made by Toucan Industries and I had one installed on my wife's Integra since they were sponsored by them. It was cable actuated so you could open it up and your exhaust was loud, but then you could close it and the exhaust would go silent. At the time, her car had a full aftermarket exhaust from Holley, headers all the way back and it was very, very loud. I installed this and when closed, her car was literally factory quiet. The downside was that when closed, it killed power since it was a huge exhaust restriction. Eventually took it off when we installed the turbo and that cut the exhaust noise a ton.
This would be a pretty extreme solution, but if you want it Prius quiet...