2008 R2 DC refuses to Sync w32tm with an external NTP server
Solution 1:
As is usually the case, posting a question usually results in my finding the answer elsewhere in somewhat short order.
The issue was that our default domain policy was being applied to the DC, and that policy configures the machine to point to time.windows.com. While I would have assumed that would work, evidently it did not. Changing all of the Windows Time Service entries to "Not Configured" and running gpupdate on the DC allowed me to set the peers correctly.