It looks like dfvn of XDA has figured out a way to get tethering enabled on HTC devices. It involves sideloading so you need an unlocked device:
1. Download XAP file here
2. Download HTC Sync for Android from HTC for USB Modem driver from HTC’s site (the whole package) or just the driver here
3. Deploy XAP file to your phone
4. Turn of your Data connection in your phone.
5. Run DFRouter and click on “HTC Modem”
6. If you only downloaded driver package just unzip to any folder, then Open Device Manager (recommended for Win7 64bit). If you installed the HTC Android package skip to step 10.
7. Find USBVID_0BB4&PID_0EFF&REV_0000&MI_01 and force update driver with HtcUsbMdmV64 (inside unzipped folder)
8. Find USBVID_0BB4&PID_0EFF&REV_0000&MI_00 and force update driver with HtcVComV64 (inside unzipped folder)
9. You will get two new device HTC USB Modem and HTC Diagnostic Interface (COM8)
10. Connect device to PC, open HTC USB Modem and select Advance Label
11. Add below command
(APN = Your carrier’s APN)
12. Create a new connection with HTC USB Modem, diag number is *99#
13.Just connect without user name and password.
It seems like some people over at XDA are having problems with this so make sure you follow the steps very carefully.