Man it would be really nice to have something better than this lame-o hand helper for the soldering. I really don't like jumping wires all over the perfboard, so I soldered some stackable headers right under the I2C port expander:

The rest of the parts showed up as well:

The relay board actually expects the pins to be pulled to ground to activate the relays. That should make initialization...interesting. Obviously don't want those relays clicking on and off while we're booting up, and the fireworks are all loaded. Speaking of fireworks, here are the igniters:

100 count for ~$25. Will need another bag or so. Ignition time at 3.3V was satisfyingly fast, fractions of a second. Jamming some green fuse in there worked like a charm, so I'm not expecting any problems at 12V.