David Damerell wrote:

However, the chain falls down the gap between the 52t and 42t chainrings,
which is a nuisance. Furthermore, I intended to replace the rear hub with
a 7-speed cassette hub; we plan to do the End to End on this tandem and I
don't expect to be able to find 6s freewheels in the back of beyond, I
have a bunch of existing 7s gear, and an 11t top sprocket would also
alleviate the gear range problems. However, I'm concerned that a narrower
chain will only exacerbate this problem.

I'm not sure what the answer is. "New cranks" is obvious, but expensive.
Returning to 48/42/28 might sort it out, but then the gearing's less
useful. Another idea that springs to mind is drilling into the 52 and 42
just below the teeth on the 42, and bolting them together - both to make
them slightly closer, and to make it impossible for the chain to descend
below that level. Grinding down the flats that engage with the spider
would also decrease the spacing, but seems pretty drastic.

The crankset may have been designed with wider, 5/6 speed chain
in mind, where the rivets stick out more. Many chainrings are
asymmetric - perhaps the 42 can be turned around, However, there
is a better solution.

I "pinned" a 52 in an attempt to mimic the Shimano pins that
assist 42-52 upshifts. I tapped small holes in the chainring a bit
below the teeth, and inserted short M5 setscrews that poke inside
by 1-2mm, just enough so the chain doesn't hit them when it's on
the 42. If you do this I think the setscrews will push the chain
inside enough to land on the 42 when downshifting.

I used a tapered tap and didn't run it quite all the way through,
so the screws are a tight fit. This keeps them from vibrating out.