I've seen this done before in order to monitor BATs (Boost Air Temperature). On my MS3 it was integrated straight into the Cobb AP and was a very useful parameter in terms of tuning.
On a WRX the only viable position I can see a sensor going is the throttle body hose, post-intercooler.
I've been looking at the Mishimoto silicone hose but am not sure if it would be a good idea to tap into it even if covered with silicone after putting the sensor in.
The only other option I can see is getting a small metal piece fabbed up, tapping that for the NPT sensor, and using two small silicone couplers for the throttle body and intercooler.
Any input or ideas for this? Not knowing that temp is driving me nuts and making tuning harder on myself and my tuner.
Gauges I am looking at:
Prosport Digital Gauge
Glowshift Analog Gauge