DOH! I just had a cosmic flash of insight, getting back to the original topic of this thread.
I already have the buck-booster which converts everything to 12V. Haven't used it yet.
With this logic relay device, I could make the buck-booster cut in at say 13.4V (battery output) and cut out if it went below 13.1V for more than 5 minutes or whatever. That way, I could get rid of the high spikes without losing so much efficiency.