‘At the moment I just use an oily wet lube on the chain when the manufacturer's lube has gone, but I might try a chain grease advertised for motorbikes. I tried that in the past on an unprotected chain, but it picked up a lot of road dirt so it was worse than an oily wet lube, it might work better than an oily lube under a chaincase’

Motorbike chains are different to ours as they have O rings keeping the original lube inside the rollers - and dirt, water and chainlube out.   Motorbike chain lube is more to lubricate the roller / sprocket interface.
Personally I think a thinner chain oil is better on a cycle chain so it can seep into the rollers (no O rings).

A better scheme if you want to get maximum miles for your money, for when you ride from home, not on tours halfway around the world, would be to have all the chains you expect to be consumed on one side of the sprocket/chainring to hand, and to fit them in order at some routine event (a distance, cleaning the chain, whatever), so that they can all wear in evenly with the gears. It has the advantage that you don't need to know with high precision how much wear per chain is optimal, you just spread it across all the chains and the metal will tell you when the limit is reached. Martin has written about this method, though not in these terms, and I think JohnR has mentioned it too.

It was worth it for me with derailleur transmissions. The simplest way was with 2 chains, take the chain off when it needed cleaning, put the other one on, then clean and lube the one taken off ready to go back on the bike at the next change. Generally at 300 to 500 km intervals, but very weather dependent. The best I managed before having to replace the cassette was with 4 chains.

Still worth it on my Bromptons - the exposed transmission close to the ground picks up more muck than on a large wheel bike.

So when should chains on Rohloff geared bikes be replaced?  So that sprockets are useable with a new chain without being flipped.

Andre says he changes his at 0.5%.
Others say 1%.

The 0.5% is only a convenience, so that I change the chain at the same time as I change oil. It fits with the concept of my near-zero maintenance bike.

A better scheme if you want to get maximum miles for your money, for when you ride from home, not on tours halfway around the world, would be to have all the chains you expect to be consumed on one side of the sprocket/chainring to hand, and to fit them in order at some routine event (a distance, cleaning the chain, whatever), so that they can all wear in evenly with the gears. It has the advantage that you don't need to know with high precision how much wear per chain is optimal, you just spread it across all the chains and the metal will tell you when the limit is reached. Martin has written about this method, though not in these terms, and I think JohnR has mentioned it too.

So when should chains on Rohloff geared bikes be replaced?  So that sprockets are useable with a new chain without being flipped.

Andre says he changes his at 0.5%.
Others say 1%.
I am aiming to change my chains at 1% but that is simply based on what I have read.

You will probably get away with 1%, but if it skipped, then the sensible thing to do would be to put the old one back on.
I still don't understand why anyone would? You're throwing away chains with a lot of wear left in order to prolong the life of cheap components. You don't have the choice with a derailleur, simply because the sprockets wear at different rtes.

In theory, with a single-speed bike, like the majority of modern hub gear bikes and virtually all Rohloff-equipped bikes, you can run a transmission set of the chainring, sprocket and chain into the ground until the first of them breaks, then replace them all at once. The benefit of this method is supposed to be that the components will all have worn together and thus fit together optimally for their state of wear until the end, the expectation being that this method would deliver the greatest possible mileage from the set and, as a bonus, avoid fitting a new chain to worn teeth, which will ensure that the new chain gets to only half the mileage of a chain fitted to new teeth or reversed gears.

"In theory" means I haven't tried it, among other reasons because the underlying assumption above is that all the components have roughly similar lifespans, which just isn't true on my bike, where the Surly stainless steel chainring will very likely have a service life in excess of a magnitude larger than the KMC X8 chains I use, and the Rohloff sprocket will have a lifespan at least 5x the chain life; in all of this the qualification is "in my zero maintenance paradigm, in which I run the chain for its entire life (to half-a-percent wear at which point, at about 4500km, I chuck it off at the Rohloff oil change) on the factory lube with nothing added, inside a Chainglider". Note also another assumption, that there is a whole link-and-a-bit chain adjustment length built into the bike, which isn't true of eccentric bottom bracket adjusters as fitted to Thorn bikes, or even on most bikes with track (slotted) frame ends at the rear of the bike, though Rohloff's own "slider" axle hangers (for which they'll give OEMs blueprints free of charge) have the "correct" length for all likely needs, including this one.