A solution is on the right side of the page. The central part of the strap, which runs under the watch body like a NATO strap, has an elastic backing, and the top piece is "ruched" to allow it to stretch AND mount to the watch pins. To expand the watch strap, you simply move the watch to more central pins, stretching the centre of the strap and increasing the length.
I've bought some cheap elastic waistband material which can double in length when stretched. A rudimentary bit of maths gives me 40% stretch possibility from the top fabric if i use semi-circular loops. With only 8 loops that results in 16mm of possible range!
I have a couple of zips on the way, and a neat trick for removing watch pins. When they arrive i'll dive into the prototyping which involves fabric, floss, and many staples!