How to protect an externally connected PT 1000 sensor
I think if you give it a unique connector that can't easily be used to apply external voltage to the sensor you'll be okay. Pt sensors are inherently isolated so they're not as fussy as thermocouples. 

Your circuit is applying an awful lot of voltage to the sensor, so it is not the best. Usually for a Pt1000 we like to see 100uA, give or take, so about 100mV. Putting 12.5x the voltage across it means 150x the self-heating, among other things.

Using a Pt100 sensor at 0.3mA would be less sensitive to moisture that might have conductive ionic contaminants, but you'd have much less output. It depends what kind of resolution and accuracy you are going for, but the self-heating is going to play havoc with readings if you are trying for high resolution.