Looks like it was the strut bearing, as we were finally able to isolate the noise.
The old parts didn't look terrible, but I honestly don't know how old that bearing was. I could get some slight movement with the part in my hands, but it wasn't "loose". Then again, the dynamics when it's installed are much different.
New bearings and seats on both corners seems to have silenced that rattle one and for all.
)