Interesting, Jonah...
I've switched entirely to flat pedals now, and was seriously considering using them in the TCR last year, if I had ridden.
For the UK I use FiveTens, which are not bad. My longest ride in them was 200km back in April, and I have to say that my feet were getting a bit sore from pedal pins by the end. I now have different pedals with slightly shorter and fewer pins and have not had such issues since, but have not ridden more than 100km since June.
Problem with the FiveTens is that they are pretty warm for hotter climates. For the TCR, I was searching for something a bit cooler. I was looking at:
- other shoes for flat pedals (
https://www.bicycling.com/bikes-gear/g20046461/the-best-mountain-bike-shoes-for-flat-pedals/)
- walking shoes along the lines of those Decathlon ones linked,
- a light pair of trainers
- maybe even sandles for south of the Alps (with socks, obvs, to avoid sunburn!).
I even bought a pair of crocs to try out (as recommended by ultralight Igor
http://ultralightcycling.blogspot.com/p/equipment-reviews_12.html) but didn't get round to it.
I think that with the right pedals, you can pretty much wear anything. You don't want the tread to be too chunky or you won't be able to fine tune your foot position so easily, which could be the issue with hiking shoes. And I think ankle support might be important, so don't want something too floppy and low cut.
Do tell me if you find the right shoes - this is a bit of a niche!