Today is a good day for all MDT 2010/2012 admins. CMTrace.exe is included in ConfigMgr 2012 SP1 Toolkit.
You don't need to get the full ConfigMgr 2012 software to find a new log viewer for the MDT log files (of course good old Trace32.exe still works, but does not exist publicly in x64 version).
Note: Even though CMTrace is available for download publicly at the Microsoft website, it is licensed to ConfigMgr clients only. However, note that with a ConfigMgr evaluation, you also get rights to use the tool (but technically/legally still bound by that evaluation license).
After installing ConfigMgr 2012 SP1 Toolkit, CMTrace.exe is found in the C:\Program Files (x86)\Configmgr 2012 Toolkit SP1\ClientTools folder.
For some odd reason, Microsoft only included the x86 executable (which extracts the x64 version when running on full Windows).
This will of course fail in x64 WinPE, in which cannot start the x86 extractor, and CMTrace.exe fails to start. This is not a problem if you have full ConfigMgr 2012, because ConfigMgr also includes a core x64 version which the ConfigMgr 2012 SP1 Toolkit does not. However, you can get it anyway. By copying the file that is extracted, and you can use Process Explorer to find it.
This is the CMTrace.exe process viewed in Process Explorer. Note the TRA1887.tmp file (may be named different on your system).
Here are the properties of the TRA1887.tmp file
Get a copy of the TRA184B.tmp file, rename it to CMTrace64.exe and add it to your WinPE Image, and voila, it works in x64 WinPE