You're not going to get core temperature in the ear but you will get solid information on how warm the body is running, the brain is a pig when it comes to blood supply.
I dunno if it's going to work but I have a $2 PIR sensor on order. No lens or board, just the sensor. I plan on masking one of the apertures and seeing if the difference reading gives me anywhere near right temperatures. I'm sure the mask will vary but I don't need high accuracy to check a house for heat leaks at night do I?