@Motley luckily a community member - @DarcRanger - has already made a smartapp for controlling the temperature via Alexa. I have confirmed it works with the Nest custom device type.
Essentially what you have to do is create two virtual dimmer switches (one for heat, one for cool), then select the switches in the smartapp along with the thermostat you would like to control. Then you expose the two virtual switches to Alexa. Name the switches heat and cool (or whatever makes sense to you) and then you can say “Alexa, set heat to XX” or “set cool to XX”.