I was going to suggest a PVC wire test (below for posterity) then I thought BBQ thermometer! One with a remote probe. I'm thinking / hoping the temperatures you really see aren't as bad as you think. You could move the BBQ probe from the outside in stopping before you get too hot for the BBQ probe.
The wire you link will work but it will be kinda stiff.
I do have an possible alternative if you go to low voltage and the PVC test isn't too toasted. Look at this wire: eBay Silicone Wire
It is good for 200°C (390 °F) it is super flexible. I have some #22 and it is literally limp. Very easy to work with.
BUT It takes time from China.
Good luck.
OLD PVC plan:
I don't know where you plan on running the wire, however assuming you are somewhat away from the flame you could install (actually only place, no connection) a piece of PCV wire. One of the single conductor wire for conduits like THHN etc. You can probably get a foot from Home Depot as a sample.
Leave it there for some time while the flame is burning then see how it fairs. I'm not suggesting it will show you that you can use PCV but it will tell you if you are close. i.e. if the PVC is not damaged too badly you would feel better with they plan you have
By way of follow up a bit. I wired in a hallogen bulb in a high temp ceramic base using 1000 degree wire.
I then got a "temp stick" which is basically a marking pencil/crayon for 900 degree's and made a few marks here and there around the inside of the fire box. The marks will turn from chalky to glossy if the temperature exceeds 900 degrees (+/- 1%).
After running for a few hours at max flame I took the front back off and noted that the marking pencil marks were still "chalky" and not glossy indicating that the areas marked did not reach 900 degree's... so I think my system will hold out... and it looks SUPER nice with the accent light on.
I am rarely this pleased about something I did!
If you look close here... what you are seeing is a reflection off the back of the fire box showing the light I added and a little shade or shield I also added along with the wires. The light is mounted pretty close to the FRONT of the fire box... near the glass but you can see it reflected on the back of the firebox if you look from way down low...
The proof is in the pudding, but I have a relay for my wood insert that kicks the fan into high gear when the exterior of the firebox top plate gets over 700F. It's a type K thermocouple with appropriate wire. I used a magnet to hold the thermocouple down that's probably only useful for its weight at this point, since I understand heat takes out the magnetism.
I unwittingly left the door open too long earlier this week, and the temp got to 820 I might see if I can set up some kind of alarm.
When we had a gas fireplace that turned on with a dumb switch, I replaced the switch with a Go-Control Z-Wave thermostat. It worked great and was tied in with my Iris system at the time. Worked with any rule that I cared to create.
I was thinking about hooking the fireplace up to HE but to tell you the truth... my setup (and experience with home automation in general) is just not reliable enough for that... I don't want to leave for the weekend and find out some glitch in the matrix turned my fireplace on while I was away.
I do like the idea of adding a thermocouple to monitor temperature in real time...
The thermostat just measures ambient temperature and turns the fireplace on and off accordingly. I do not know of a way to measure thermocouple temp. If I did I would use it to monitor my gas water heater. It does blow out on occasion.
Edit: I see that that question was not directed to me.
To read a thermocouple, an amplifier is needed to increase the voltage so that an MCU can read it.
The amplifier come in form of IC/modules. One of them is AD8495 which is linked by @JohnRob. I personally tested with my own homemade Zigbee sensor. One of the expansion is Analog input that can read from the AD8495. Once you got a voltage, you can use a simple formula to get the temperature.
If you interested on what I have done with my Zigbee module, I am happy to help you set one up. I have helped a few members here in the past to get one running in their home. There is need to write code. But, you do need to connect wire from the Zigbee Module to the AD8495.
Another path is to use Arduino. @ogiewon can help set one up for you. You can get the same AD8495 and wire on to one of the Analog input of the HubDuino.
If you need help, please feel free to reach out. I will be happy to help.
I might see if I can set up some kind of alarm.
