I poked a bit on my phone. It's the international version, HBOOT 1.30.0000 CID HTC__001.
Anyway, there's a fastboot command called "fastboot oem enableqxdm".
QXDM is a piece of software designed to talk directly with the modem. Has anyone looked at that avenue? If not, I intend to right now.
I'll post findings when I can.
---------- Post added at 04:15 PM ---------- Previous post was at 03:46 PM ----------
OK, turns out, (at least with my setup), the phone WILL allow you to enable QXDM mode. The command "fastboot oem enableqxdm 1" completes successfully.
Now I've just gotta get the phone into diagnostic mode. On CDMA phones, that's ##3424#, which obviously doesn't work on the One X+.