Not sure that the heater for the batteries is all that large, plus, you're trying to warm 450# of metal, liquid, and batteries...it takes time. I do not believe you'd gain much of anything in a 1/2-hour. As early as you can, set a departure time, and you'll get as much as possible. SO, if you decided you were leaving in 1/2-hour, and it was 1630, set it to 1700 and you'll get what you get...not much, but anything is better than nothing. It might take you an extra 30-seconds to do that verses the cabin preconditioning, but it's not particularly hard or time-consuming.
Just the process of discharging the batteries, say to precondition the cabin, will start to warm them, even without setting the departure time. Done while on the EVSE, you'd still be gaining a bit.