In my Jeep, my transponder is in the upper left corner of the windshield, as high as I can get it. They recommend it in the lower corner but my inspection sticker is there and I didn't want to block any more view by placing it above the sticker. In the upper corner it is out of my view and has worked fine going thru the booths. Being a bit shorter in stature than most people, the items mounted high on the windshield (toll tag, rear view mirror, etc.) have not been much of a problem for me.
As far as mounting under the 'hood' in the front:
I believe the sensors in road at the booths have to 'see' a vehicle before registering the tag. If it is mounted to far forward on the vehicle where the scanner reads the tag before the front tires trigger the road sensor, it may not read. I could be wrong on this and possibly the Smart would not be an issue since it is so short anyway.
Maybe you can mount it up front temporarily as an expirement and go thru a manual lane to see if it reads, that way if it doesn't, you can pay at the booth and not worry about a ticket.
Good luck and let's be careful out there,
Pops and Car-lito (eventually)