After much hair-pulling and frustration, I've determined that calling
document.location.reload()
from within an instance of WKWebView on iOS 9 Beta 1/2, immediately crashes the app with the following symbolicated stack-trace: (imgur link just in case: http://imgur.com/UGIVgM9)
Sorry for the image rather than text, as I wasn't able to see anything but [redacted] for each line in the trace.
Also, something to note:
window.location.href = window.location.href
Does in fact work without a crash, while effectively doing the same thing, without passing along POST data.
Let me know if anyone else has seen this behavior!