This matches directory structure used commonly in Linux-land, and it's cleaner than mixing overlays into the existing module paths. Overlays are still mixed in by specifying fdt_overlays in loader.conf(5).
As an aside, I'm contemplating opening this up to eventually allow some kind of "auto-discovery" mode of overlays, where it will enumerate all overlays in /boot/overlays and apply any that are applicable to the board we've booted on. This would be helpful to me, at least, to forego editing of /boot/loader.conf every time I want to add an overlay. My serial adapters are less than great and editing a file over these things is particularly painful. =)
I have some WIP to honor /compatible on overlay if it's specified, which would then be compared with /compatible on base FDT to know in advance if we should bother trying to apply it. I -think- the /compatible checking should go in regardless of auto-discovery; every overlay example I found seemed to expect that /compatible in an overlay mattered.