+1 please Steinberg. This would be an excellent ‘pro’ feature I think. The log should be saved with (or in) the project file.
Years back I wrote a plugin like this but it’s not compatible now 
It did everything the HOFA plugin does but also when inserted in the main output bus it used the audio level passing through to guess whether you were really using cubase or whether you’d left it idle and gone for lunch (i.e. no audio, stop the clock). While it worked it was very useful for estimating how much time a session had actually taken.
Another tool I once wrote looked at the save history of my projects (I always save as a new version) including the autosave backups and did a rough calculation to estimate session time over the project history… It definitely wasn’t an exact science but it gave me a rough idea, certainly of the dates.
Mike.