There is already a separate thread on the issue of AWD online, SKOT_FREE, try searching for "Why is 10-90 frowned upon".
While we encourage drifters of all stripe, it is more convenient for drift competitions, freestyle and tandem if all participants are Rear wheel drive.
The problem with engine swaps and drivetrain swaps is that, realistically, anything is possible in real life, given enough money and fabrication. V8 Miata? Done. One of the members here even works for the shop that sells the conversion. V8 RWD conversion for a compact hatch? KugelKomponents sells the kits. Big V8 in the front end of a microcar? Been done... You simply cut out the front firewall and cut a hole in the front grille for the rest of the engine. Not pretty. Drives even worse than it looks.
Possibly a little more manageable if GT5 simply includes popular swaps as separate cars for DLC. Though full differential control for AWDs should be enough to keep people entertained for a long time.