Or a Surly Dingle cog. Plus 2 different chainrings. Means you can have 2 gears, while keeping the same chain length.
Not a silly idea. A century ago the practice was current, usually involving a double-sided hub with two double freewheels, on each side. Phillipe Marre used this system along with a two speed dérailleur and two chainrings to get 8 different gears.
I have one bike with a single pulley chain tensioner. It looks pretty but i wouldn't want to take it anywhere mission critical, it has already lost the pulley once (fortunately when I was very nearly home). The only reason I use it is that the frame it is on has vertical ends).
If I really wanted a bike to go into places where mechanical misfunction was going to be very inconvenient I would probably avoid any mechanical gear changer. Depending on what was available from the BMX and trials worlds it would probably be a double sided hub with a 3sp freewheel (16-22 should be available fairly easily) and on the other side the biggest single freewheel that I could source. Another solution would be a disc cassette hub with 3 sprockets on a freebody one side and a single fixed cog fastened to the disc fitting the other side. Match all that to the longest horizontal ends around to give a good range of adjustment and three chainrings to match up to the sprockets. Just about bomb proof.
Decathlon did do a very cheap ss bso mtb. It cost under 100€. Unfortunately also very heavy IIRC but unlikely to get pinched (thieves wouldn't be strong enough to pick it up). It used to have all the rack points and like that one would want for a tourer. There should be a few knocking about in the rivers that cross Rennes (make quite good anchors I would think!!)