I ended up picking up a heater Walmart had on sale, rather than the one I wanted on Amazon. On its low setting it draws ~600w, but it has a thermostat and shuts off if tipped over, so I don't have to worry about melting the car. Still, I plan to watch it closely for a few days before I let it run unattended on the timer.
The existing block heater is 400w, which means with the other heater running I have about 800w in total to play with while staying within the 15 amp rating of my programmable power strip and outlet. I'm thinking it would be wise not to approach it too closely, so I'm still debating on two 150w heaters (one for the oil pan and one for the transmission) or a single 500w for the oil pan.
Here's what I have set up so far:
WiFi power strip, programmed to come on 3 hours before my alarm goes off and shut off at 8AM:
12v trickle charger (also on the timer for now, for simplicity's sake), 400w block heater and 600w cabin heater all meet here:
Outlet I ran through the firewall:
Small electric heater:
I might end up running several extension cords out, since I'd like the heater to be on a different timer than the block heater (ideally it should come on when I wake up, not before), and the 12v tender to run all the time.