No. It is NOT possible.
You can only designate the car class, can NOT choose a model, grade, mileage, GPS models, color, model year, nor further car details etc., due to class specification reservation.
However, car models can be specified for the below car rental companies.
Nissan;
Car model specification is possible when you book.(there is only 1 model car per 1 class).
*There is a case another model car is offered including an upgrade due to irresistable reasons.
*Others;
Nippon (Hokkaido), Niconico (Chiba), Avis (Kyushu), Budget (Okinawa), Kuruka (Okinawa), Orix (Kansai Airport, Osaka) etc.