Duct tape as shoe liner? Bad idea?

If you sandpaper the bottom a bit, I bet it'll hold. Dr. Schol's insoles also have a rubber bottom on them that sticks pretty well. You could also just wear them in streams and superglue the insole in.

Duct tape might bunch up as the adhesive starts to pull away from the fabric of the tape. I experienced this when I taped some tubes to my bike before a long trip- as the tape started to stretch back, it just dragged the adhesive around. In your flexible shoe, it will likely pull away from the insole and then bunch under your feet, causing blisters and a nasty scenario where you have to peel it off and then wear shoes filled with adhesive and sticky socks for a week.

On an extended hike, I noticed that I was starting to develop a raw spot on my foot where the insole met the shoe wall. There was gap between the insole and the shoe wall, and it was chaffing my skin, so I bridged the gap with duct tape. No problems after 300 miles or so. Get ready for a lot of residue left over though. Once you put duct tape in there, you're always going to need to put something over it, unless you want your socks to be sticky too.