@XaiaX it seems to have an extremely low vapor pressure at room temperature (which isn't that surprising, given that it boils at nearly 200 deg C). You might have better luck trying to create a very fine caffeinated mist (which will quickly turn into caffeinated dust), which would happen with some ultrasonic humidifiers.

All that said, trying to absorb something that's a poison at achievable doses via atypical means makes me wary (for one specific thing, I am not sure how this affects uptake rate, but I'm more worried about unknown unknowns).