To enable private context tracing in Desktop Execution Trace Toolkit, in the file %LocalAppData%\National Instruments\Desktop Execution Trace Toolkit\DETT.ini place the following:
[Preference]
ShowPrivateContext="True"
It's important to ensure DETT is closed down when you change the file; on exit, DETT flushes to file it's currently loaded settings.
(Placing this info here with some searchable terms, to save my future self that infrequent 20-minute spelunking exercise of finding it again)