@Sebkinne Awesome, got it! I now see ./bin/ar71xx/openwrt-ar71xx-generic-*-{factory,sysupgrade}.img , which is great, though I'm just interested in replacing the kernel.
I tried to scp kernel_3.18.19-1-f348b1f5b567b17ca4353ee14083f045_ar71xx.ipk to my Pineapple and ran
$ opkg install --force-downgrade kernel_3.18.19-1-f348b1f5b567b17ca4353ee14083f045_ar71xx.ipk
but my system is still using the old kernel... which might be a good thing, because that .ipk file is just 814 bytes -- clearly too small to be a legit kernel!
How can I just replace my kernel? With one of these files, perhaps?
./openwrt-ar71xx-generic-vmlinux.lzma
./openwrt-ar71xx-generic-vmlinux-lzma.elf
./openwrt-ar71xx-generic-vmlinux.gz
./openwrt-ar71xx-generic-vmlinux.elf
./openwrt-ar71xx-generic-vmlinux.bin
Thanks!