This is the type of stuff I was thinking of...
https://www.austrotherm.com/en/main6/...29/index.shtml

There's stuff on a roll like that in Home Depot though, but can't find a good pic or description online...

I figure you can spray mount that then paint over it. Flock paint it even.

You may want to consider Spectrum by Second Skin. It isn't cheap, but it works very effectively from everything I've heard.

I used their Damplifier mats in both my Comet & my xB and it did an amazing job of reducing noise. I have something like 40 sq ft of the stuff in the xB, and the road noise & engine drone is way down, and it added under 20lbs to the weight of the car.

That stuff looks fantastic, but do you think it would be durable enough to hold up to daily foot traffic on, say, the driver's side?

No idea, I've never used spectrum myself, sorry. However, there is a forum on their site, and if you ask there you will likely get responses from both employees and users of their products.

...and that's where you add a layer of bedliner stuff. You can get it in a spray can at WalMart, and that ought to be enough to toughen a couple key spots.