I'm in the process of finalising a production PCB design and designing a 3D fan housing model (similar to the one in this post) which would be able to hold the circuit board securely, I think the best place to locate the board (and therefore sensor) would be horizontally just above the fan, near the flyback cage, and in the 'dead' air space just below the top of the case, where the internal temps are likely to be highest.
I've also designed a test circuit and written a separate program for testing, which I'm in the process of wiring up to the Mac's serial port, so I can gather and save data to plot internal temp / fan PWM / fan RPM over time, which may be interesting for the community. Just need to wire it all up and spend some time capturing and plotting the data... I also plan to make some recordings, to compare the sound levels from stock fan / Noctua at full speed, and what the fan sounds like at different temperatures.
I'm thinking of making a small run of these boards and could make them available for a reasonable cost (£15 or less, depending on number).
Do message me if you're interested so I can gauge whether to order some parts for more than just me! Be aware though that this design requires a 4-pin PWM fan rather than the 3-pin style (which will always run at full speed if connected to the Mac's 12V supply).