Ok this is surprisingly simple. I found this thread where what's needed is actually explained in detail: https://betest.freeflarum.com/d/61-h3-news/9
Then I found a target I'd overlooked which does what we need: ninja -f build.ninja.h3 basic_sd.img
This seems to be running dd to stitch together the various pieces of the image:
dd if=/dev/zero of=basic_sd.img.fs bs=1M count=60 ; mkfs.vfat -F 32 basic_sd.img.fs ; dd if=/dev/zero of="basic_sd.img" bs=1M count=4 ; cat basic_sd.img.fs >>"basic_sd.img" ; rm -f basic_sd.img.fs ; parted -s "basic_sd.img" mklabel msdos ; parted -s "basic_sd.img" mkpart primary fat32 4 67 ; dd if=/home/parallels/devel/orangepi/allwinner-bare-metal/sunxi-spl.bin of="basic_sd.img" conv=notrunc bs=1024 seek=8 ; dd if="basic.uimg" of="basic_sd.img" conv=notrunc bs=1024 seek=40 ; test -e "init_dir" && mcopy -s -i basic_sd.img@@4M init_dir/* ::
Anyway .. it's done, and it boots, and I tweaked a couple of things I'm going to play with: firstly I increased number of backgrounds to 6, and secondly I upped the amount of pixel RAM so PSIZE(2) is now 5 times the screen Y rather than just 2 (so I can fit more sprites in). Will see how this goes 😀