Like I already pointed out, there are in fact cars with transversely mounted engines and awd drivetrains. Case in point being the EVO. The Focus WRC and probably several other cars on the WRC circuit also have transversely mounted drivetrains. Mainly this is because they are based on fwd car platforms, but they still manage to perform slightly better than alright.

I would surmise that much of the engineering of the RL, while not necessarily on the global midsize platform, is in fact common. Honda's sole experience with building awd vehicles, including ones with V6 engines (Pilot, MDX) is with transversely mounted engines. There is a total of one current Honda with a longitudinally mounted engine, which is the S2000. There were several fwd cars with longitudinally mounted engines: Vigor, 1st gen TL, 2nd gen Legend, 1st gen RL. It would stand to reason to me, that the longitudinal mounting scheme was not retained because of packaging concerns with the addition of the awd system.

Also as far as efficiency is concerned, look at how the driveline is set up. Power is not sent to a transfer case and then back out to each wheel: it is sent to the front wheels first and then the rear wheels sort of siphon it off of that path. In effect it's no less efficient even though the engine is mounted sideways.