Tapping 100A feeder at meter for EV charger
You know you can't tap arbitrarily, but you can tap when the tap is the same ampacity as the trunk.  Simply because the breaker protecting the trunk is able to protect the tap.  

The problem is, you're hanging your hat on that, and your hat will fall down when you go to 200A service later.

So the first part here is some vision work, figuring out how you're going to protect that 100A branch on  the happy day 200A arrives.  I would propose some sort of combination meter pan + mini-panel with a main breaker and a couple of spaces for 2-pole breakers for branches.  Just have that product in your mind's eye (i.e. Selected and priced) for now. 

In the garage, I doubt the EV charger is rated for a 100A feed. that means you will need a subpanel - but any cheapie will do for a temporary.  It simply needs to house a breaker of the ampacity specced by the charger. 

I take it the 100' wire run will be a keeper, take super care to leave yourself excess length any way possible.  Aluminium is your friend on a run like this.  

You won't need to upsize the wire for distance.  If you plan to run the garage circuit to full 100A actual draw on a regular basis, lay 120/125A wire.