Sounds possible, it would be a good idea. Not many people will care for loads of power, just fuel efficiency, decent power, and still the same lexus experience
The bottom of the 2.2 engine is the much the same on both but the cylinder heads are quite different. On the FTV (150bhp) the injectors are under the cam cover but on the FHV (179bhp) they are much more exposed and as Stewart says they are piezo electric on the FHV and solenoid on the FTV. The cylinder head also has a port for a fifth injector which sprays diesel into the exhaust to stabilise emmissions. The inlet and exhaust systems are different but that is mainly in conjunction with the D-CAT system which is designed to lower emmisions after the power uprating.
except for in Europe. I dont expect a RC200d anywhere near North America, but as said in the OP its nearly mandatory to be competitive in Europe...a diesel coupe is a super niche market