Your idea of a downward-pointing MLX90614 just might work. I tried some experiments with an MLX90614 on the floor, pointing directly upwards, monitoring both the device body temperature and the sensor temperature.
- After a few minutes the readouts settled down to about 22.6 +/- 0.1 C (sensor) and 22.5 +/- 0.1 C (device body).
- If I put my hand directly over the sensor at about 5 cm distance, it reads out about 32 C
- If I put my hand directly over the sensor at about 1 m distance (it sees part of my bare arm too), it reads out about 23.2 C
- If I stand up, with my feet about 20 cm from the sensor, it reads out about 23.4 C.
In all of the above the device body temperature remained at 22.5 +/- 0.1 C. So, you might expect about 0.5 - 1.0 degree C increase if a person approaches, but clothing will obviously affect the readings. You will just have to try the experiment yourself.