I have a billowy smoke pyro sim cached out. I'd like to be able to have it fade in rather than just be immediately present at the surface of the emitter. If this was particles I could use chramp and @nage to ramp in the alpha, but I don't know how to solve this as a volume.

You could have it fade in from the proximity to your emitter with xyzdist(). Or you could also emit temperature in a rapidly cooling sim, and have the density increase when the temperature decreases.

I'd try to do this in shading rather than my sim. Get the pyro sim you're happy with and then animate the density in the shader to fade it in. It'll be faster to iterate through timing and speed variations.

you can potentially have another field that's sourced at 1 and decays over time (similarly to what temperature does, or hijack temperature if it's already doing it in your setup)
then use it's values and remap to the fade mask you can multiply your density with, which will fade the density

also watch this, where Attila does something similar to control base fading on of the fire
https://vimeo.com/739683075 [vimeo.com]