Asked the same thing on Reddit. Solution copied below:
Solved thanks to /u/Madgemade
It is available in afterbuner. In monitoring, scroll almost right to the bottom, you will see "GPU Temperature 2", that's the junction temp.
Tried it, it works. Also holy Land Grid Array, the difference in temperature unreal. 65°C edge / 95°C junction @ 1765mhz undervolted to 1062mV.