That’s as far as I’ve got with this thought experiment. What are your thoughts? Is it just a plain stupid idea, or have my ideas sparked a glimmer of neurological activity? Please do add your thoughts and let see where this goes.
I think that's basically what a Streetmachine (and other bikes in the same mould from Azub and similar) is; being a highly stable expedition tourer designed to cope with unmade roads. Upright riding position means you can see the ground. USS means you can sit forward and shift your bodyweight a bit on the technical bits, which I think is more useful than keeping the width down, except when dealing with Silly Sustrans Gates™.
The 20" front wheel is a compromise, but you need to be able to get a foot down, so riders of less than Dutch proportions don't have a choice. 24" might be possible, though, as presumably they make half-decent off-road tyres in that size for children's bikes.
I believe Ben of Kinetics fame once built up a spare Streetmachine frame (without all the racks and things) as a single-speed off-roader, and it worked surprisingly well. Can't find the page he wrote about it.
I'd put the ICE Adventure in the same category, though obviously trikes aren't so good on singletrack. The ICE Full Fat is a serious off-roader for mad people.
In practical terms, I'll happily ride gravel tracks on my Streetmachine, often with the full set of luggage. What makes me nervous is mud. Marathons have their limits, and if I'm serious enough about off-roading to use knobbly tyres I'll use my mountain bike. While being able to handle some mild off-road is important for a touring 'bent, I still think DF bikes are the best solution for off-road riding.