How many TETRIX controllers can you control from a single NXT sensor port? The ROBOTC motor and servo configurator appears to support 4 per port. Is that a technical limit of the controllers and the NXT? Or is that limit a result of the configurator or some aspect of robotc?

Is it possible to use 5 TETRIX controllers off of a single sensor port?

Natively ROBOTC only supports daisy-chaining of up to 4 Motor or Servo Controllers on a single port. If you wish to use more than 4 per port, I would suggest contacting the manufacturer of the Motor and Servo Controllers, HiTechnic (http://www.hitechnic.com/contact) to see if this is possible.

You can write your own I2C drivers in ROBOTC, so if you are able to connect more than 4 devices to a port you should be able to code it using custom I2C calls; again, I would suggest contacting HiTechnic to see what they recommend first.

How about attaching that 5th controller to a sensor mux port? Seems like that might be do'able if you write your own I2C code, and it would avoid having to dedicate a second NXT sensor port exclusively for controller(s).

No, my understanding is that Sensor MUX is for sensors only (i.e. can only read, not write). So there is no way you can connect a motor controller to it because I would imagine you must have to write the motor value to it.