The porting of the Transterpreter is straight-foward. You check the sources out, configure your cross-compiler, and go. Some twiddling with 'make' is usually the biggest problem we run into.
Porting to the NXT will be slightly less straight-forward.
The core Transterpreter will cross-compile to the ARM7 without any problems. However, there's a little problem of bringing up the hardware on the NXT that must be addressed. From my experience working on the Tmote Sky, the OS developer needs to check and set a significant number of pins to bring up each piece of hardware. These bit-twiddling operations are tricky; for this reason, it is likely that we'll want to get a JTAG reader for the Mac and be able to inspect the hardware while developing.
The Atmel AVR is likely to be the first device we bring up; it gives us access to the sensors, motors, and buttons on the device. Once we have rudimentary access to the NXT's sensors and motors, the simplest way to get feedback from our programs will be to implement sound. In increasing order of difficulty, I suspect the display, the USB port, and Bluetooth radio will follow. Once we get the low-level bindings in place, however, we should be able to write the majority of the NXT port in occam-pi or 42, which will make development a good deal more pleasant.