I think it would be possible to have a 12V Battery maintainer programmed to turn on for a short period (1/2 hour ?) whenever the vehicle is turned off . The DC-DC converter would need to be on during this .
The high-voltage system remains on for at least 30 minutes after a door, the frunk, or the hatch is unlocked or opened. The high-voltage system also remains on for a while after an i3 is shut off (not sure how long). If the 12 V battery's voltage is below a certain limit, the DC-DC converter will turn on to charge the 12 V battery during these periods. So the 12 V battery can be charged after an i3 is shut off without the need for an additional maintainer circuit.
What I'm thinking of is a maintenance cycle . Low current battery maintainers or tenders have a mode that helps in the longevity of the battery . Desulfation is one word that I've seen , I'm not sure that it is applicable to our 12V battery though . But improving the battery's longevity would mean a lower frequency of replacements . A conflict of interest .