A solution that seems to work is to create a new .bbclass inherited from my layer's linux .bbappend file that implements uboot_prep_kimage:append() to append the DTB to linux.bin. This code would run at the beginning of do_uboot_mkimage() before the uImage is created.

There are a couple of prerequisites such as setting KEEPUIMAGE = "yes" and KERNEL_DEVICETREE_BUNDLE = "0".

https://gist.github.com/rgov/0628785685ab858a99c5bfca626c1d8f

I would still be curious to know why KERNEL_DEVICETREE_BUNDLE doesn't work as expected if that is supposed to be the supported method of achieving this, but my solution at least lets me move on.