Quite easy actually. At least for standard freezeouts.
You will need to wait until the late registration is over though:
Number of players registered * Starting stack = Total Chips in Play
Total Chips / 40 = BB when HU is most likely reached (since players will be in HU more often then not when the average stack hits 20BB)
Now you will have to look at the blind levels and see when the according blind level is reached (when your result matches the BB). Be aware of the 5 minute breaks every hour.
For rebuys you would need to wait until after the addons. Then you can do the same calculation using
Number of players * Average Stack in order to get to the total chips in play.
The result is not perfect but it should get quite close to the final result if there are no deals or such.
Regards,
Asaban