So, it turns out the hole pattern isn't that oddball. It seems to be simply 5x110, but one of the threaded holes on the tone ring is drilled off-center. This caused a bunch of headaches until I figured it out. Always nice when OEM parts have issues.
Drawing is attached. You might be able to tighten up the radii of the bolt holes a bit, but it all depends on what type of tolerance for part imperfections you want. Thickness and countersinking would depend on your material and application, so I left that out of the drawing. The inner and outer radii of the adapter itself don't need to be exact, and might actually vary depending on your hub and rotor. Basically, everything in here other than the 5-hole patterns aren't test fit for other applications, so use without verifying at your own risk.
SubaruToneRingAdapter.pdf