It doesn’t seem like this is what you are asking, but I feel I should put this here anyway for those that are curious.
You can tell how long a client has had the BES Client installed by the following:
minimum of subscribe times of sites
So if you wanted an alert of newly installed clients, you could have a web report that looks for and alerts:
(now - RESULTS_FROM_THE_ABOVE_PROPERTY) < 1*day